This patch series adds the live migration cpr-exec mode.  In this mode, QEMU
stops the VM, writes VM state to the migration URI, and directly exec's a
new version of QEMU on the same host, replacing the original process while
retaining its PID.  Guest RAM is preserved in place, albeit with new virtual
addresses.  The user completes the migration by specifying the -incoming
option, and by issuing the migrate-incoming command if necessary.  This
saves and restores VM state, with minimal guest pause time, so that QEMU may
be updated to a new version in between.

The new interfaces are:
  * cpr-exec (MigMode migration parameter)
  * cpr-exec-args (migration parameter)
  * memfd-alloc=on (command-line option for -machine)
  * only-migratable-modes (command-line argument)

The caller sets the mode parameter before invoking the migrate command.

Arguments for the new QEMU process are taken from the cpr-exec-args parameter.
The first argument should be the path of a new QEMU binary, or a prefix
command that exec's the new QEMU binary, and the arguments should include
the -incoming option.

Memory backend objects must have the share=on attribute, and must be mmap'able
in the new QEMU process.  For example, memory-backend-file is acceptable,
but memory-backend-ram is not.

QEMU must be started with the '-machine memfd-alloc=on' option.  This causes
implicit RAM blocks (those not explicitly described by a memory-backend
object) to be allocated by mmap'ing a memfd.  Examples include VGA, ROM,
and even guest RAM when it is specified without without reference to a
memory-backend object.   The memfds are kept open across exec, their values
are saved in vmstate which is retrieved after exec, and they are re-mmap'd.

The '-only-migratable-modes cpr-exec' option guarantees that the
configuration supports cpr-exec.  QEMU will exit at start time if not.

Example:

In this example, we simply restart the same version of QEMU, but in
a real scenario one would set a new QEMU binary path in cpr-exec-args.

  # qemu-kvm -monitor stdio -object
  memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/ram0,share=on
  -m 4G -machine memfd-alloc=on ...

  QEMU 9.1.50 monitor - type 'help' for more information
  (qemu) info status
  VM status: running
  (qemu) migrate_set_parameter mode cpr-exec
  (qemu) migrate_set_parameter cpr-exec-args qemu-kvm ... -incoming file:vm.state
  (qemu) migrate -d file:vm.state
  (qemu) QEMU 9.1.50 monitor - type 'help' for more information
  (qemu) info status
  VM status: running

cpr-exec mode preserves attributes of outgoing devices that must be known
before the device is created on the incoming side, such as the memfd descriptor
number, but currently the migration stream is read after all devices are
created.  To solve this problem, I add two VMStateDescription options:
precreate and factory.  precreate objects are saved to their own migration
stream, distinct from the main stream, and are read early by incoming QEMU,
before devices are created.  Factory objects are allocated on demand, without
relying on a pre-registered object's opaque address, which is necessary
because the devices to which the state will apply have not been created yet
and hence have not registered an opaque address to receive the state.

This patch series implements a minimal version of cpr-exec.  Future series
will add support for:
  * vfio
  * chardev's without loss of connectivity
  * vhost
  * fine-grained seccomp controls
  * hostmem-memfd
  * cpr-exec migration test


Steve Sistare (26):
  oslib: qemu_clear_cloexec
  vl: helper to request re-exec
  migration: SAVEVM_FOREACH
  migration: delete unused parameter mis
  migration: precreate vmstate
  migration: precreate vmstate for exec
  migration: VMStateId
  migration: vmstate_info_void_ptr
  migration: vmstate_register_named
  migration: vmstate_unregister_named
  migration: vmstate_register at init time
  migration: vmstate factory object
  physmem: ram_block_create
  physmem: hoist guest_memfd creation
  physmem: hoist host memory allocation
  physmem: set ram block idstr earlier
  machine: memfd-alloc option
  migration: cpr-exec-args parameter
  physmem: preserve ram blocks for cpr
  migration: cpr-exec mode
  migration: migrate_add_blocker_mode
  migration: ram block cpr-exec blockers
  migration: misc cpr-exec blockers
  seccomp: cpr-exec blocker
  migration: fix mismatched GPAs during cpr-exec
  migration: only-migratable-modes

 accel/xen/xen-all.c            |   5 +
 backends/hostmem-epc.c         |  12 +-
 hmp-commands.hx                |   2 +-
 hw/core/machine.c              |  22 +++
 hw/core/qdev.c                 |   1 +
 hw/intc/apic_common.c          |   2 +-
 hw/vfio/migration.c            |   3 +-
 include/exec/cpu-common.h      |   3 +-
 include/exec/memory.h          |  15 ++
 include/exec/ramblock.h        |  10 +-
 include/hw/boards.h            |   1 +
 include/migration/blocker.h    |   7 +
 include/migration/cpr.h        |  14 ++
 include/migration/misc.h       |  11 ++
 include/migration/vmstate.h    | 133 +++++++++++++++-
 include/qemu/osdep.h           |   9 ++
 include/sysemu/runstate.h      |   3 +
 include/sysemu/seccomp.h       |   1 +
 include/sysemu/sysemu.h        |   1 -
 migration/cpr.c                | 131 ++++++++++++++++
 migration/meson.build          |   3 +
 migration/migration-hmp-cmds.c |  50 +++++-
 migration/migration.c          |  48 +++++-
 migration/migration.h          |   5 +-
 migration/options.c            |  13 ++
 migration/precreate.c          | 139 +++++++++++++++++
 migration/ram.c                |  16 +-
 migration/savevm.c             | 306 +++++++++++++++++++++++++++++-------
 migration/savevm.h             |   3 +
 migration/trace-events         |   7 +
 migration/vmstate-factory.c    |  78 ++++++++++
 migration/vmstate-types.c      |  24 +++
 migration/vmstate.c            |   3 +-
 qapi/migration.json            |  48 +++++-
 qemu-options.hx                |  22 ++-
 replay/replay.c                |   6 +
 stubs/migr-blocker.c           |   5 +
 stubs/vmstate.c                |  13 ++
 system/globals.c               |   1 -
 system/memory.c                |  19 ++-
 system/physmem.c               | 346 +++++++++++++++++++++++++++--------------
 system/qemu-seccomp.c          |  10 +-
 system/runstate.c              |  29 ++++
 system/trace-events            |   4 +
 system/vl.c                    |  26 +++-
 target/s390x/cpu_models.c      |   4 +-
 util/oslib-posix.c             |   9 ++
 util/oslib-win32.c             |   4 +
 48 files changed, 1417 insertions(+), 210 deletions(-)
 create mode 100644 include/migration/cpr.h
 create mode 100644 migration/cpr.c
 create mode 100644 migration/precreate.c
 create mode 100644 migration/vmstate-factory.c

-- 
1.8.3.1

On Mon, Apr 29, 2024 at 08:55:09AM -0700, Steve Sistare wrote:
> This patch series implements a minimal version of cpr-exec.  Future series
> will add support for:
>   * vfio
>   * chardev's without loss of connectivity
>   * vhost
>   * fine-grained seccomp controls
>   * hostmem-memfd
>   * cpr-exec migration test

Another request besides the questions I threw already.. could you push to a
tree where it has everything?  Maybe that'll help to review also the min set.

Thanks,

-- 
Peter Xu

Steve Sistare <steven.sistare@oracle.com> writes:

> This patch series adds the live migration cpr-exec mode.  In this mode, QEMU
> stops the VM, writes VM state to the migration URI, and directly exec's a
> new version of QEMU on the same host, replacing the original process while
> retaining its PID.  Guest RAM is preserved in place, albeit with new virtual
> addresses.  The user completes the migration by specifying the -incoming
> option, and by issuing the migrate-incoming command if necessary.  This
> saves and restores VM state, with minimal guest pause time, so that QEMU may
> be updated to a new version in between.
>
> The new interfaces are:
>   * cpr-exec (MigMode migration parameter)
>   * cpr-exec-args (migration parameter)
>   * memfd-alloc=on (command-line option for -machine)
>   * only-migratable-modes (command-line argument)
>
> The caller sets the mode parameter before invoking the migrate command.
>
> Arguments for the new QEMU process are taken from the cpr-exec-args parameter.
> The first argument should be the path of a new QEMU binary, or a prefix
> command that exec's the new QEMU binary, and the arguments should include
> the -incoming option.
>
> Memory backend objects must have the share=on attribute, and must be mmap'able
> in the new QEMU process.  For example, memory-backend-file is acceptable,
> but memory-backend-ram is not.
>
> QEMU must be started with the '-machine memfd-alloc=on' option.  This causes
> implicit RAM blocks (those not explicitly described by a memory-backend
> object) to be allocated by mmap'ing a memfd.  Examples include VGA, ROM,
> and even guest RAM when it is specified without without reference to a
> memory-backend object.   The memfds are kept open across exec, their values
> are saved in vmstate which is retrieved after exec, and they are re-mmap'd.
>
> The '-only-migratable-modes cpr-exec' option guarantees that the
> configuration supports cpr-exec.  QEMU will exit at start time if not.
>
> Example:
>
> In this example, we simply restart the same version of QEMU, but in
> a real scenario one would set a new QEMU binary path in cpr-exec-args.
>
>   # qemu-kvm -monitor stdio -object
>   memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/ram0,share=on
>   -m 4G -machine memfd-alloc=on ...
>
>   QEMU 9.1.50 monitor - type 'help' for more information
>   (qemu) info status
>   VM status: running
>   (qemu) migrate_set_parameter mode cpr-exec
>   (qemu) migrate_set_parameter cpr-exec-args qemu-kvm ... -incoming file:vm.state
>   (qemu) migrate -d file:vm.state
>   (qemu) QEMU 9.1.50 monitor - type 'help' for more information
>   (qemu) info status
>   VM status: running
>
> cpr-exec mode preserves attributes of outgoing devices that must be known
> before the device is created on the incoming side, such as the memfd descriptor
> number, but currently the migration stream is read after all devices are
> created.  To solve this problem, I add two VMStateDescription options:
> precreate and factory.  precreate objects are saved to their own migration
> stream, distinct from the main stream, and are read early by incoming QEMU,
> before devices are created.  Factory objects are allocated on demand, without
> relying on a pre-registered object's opaque address, which is necessary
> because the devices to which the state will apply have not been created yet
> and hence have not registered an opaque address to receive the state.
>
> This patch series implements a minimal version of cpr-exec.  Future series
> will add support for:
>   * vfio
>   * chardev's without loss of connectivity
>   * vhost
>   * fine-grained seccomp controls
>   * hostmem-memfd
>   * cpr-exec migration test
>
>
> Steve Sistare (26):
>   oslib: qemu_clear_cloexec
>   vl: helper to request re-exec
>   migration: SAVEVM_FOREACH
>   migration: delete unused parameter mis
>   migration: precreate vmstate
>   migration: precreate vmstate for exec
>   migration: VMStateId
>   migration: vmstate_info_void_ptr
>   migration: vmstate_register_named
>   migration: vmstate_unregister_named
>   migration: vmstate_register at init time
>   migration: vmstate factory object
>   physmem: ram_block_create
>   physmem: hoist guest_memfd creation
>   physmem: hoist host memory allocation
>   physmem: set ram block idstr earlier
>   machine: memfd-alloc option
>   migration: cpr-exec-args parameter
>   physmem: preserve ram blocks for cpr
>   migration: cpr-exec mode
>   migration: migrate_add_blocker_mode
>   migration: ram block cpr-exec blockers
>   migration: misc cpr-exec blockers
>   seccomp: cpr-exec blocker
>   migration: fix mismatched GPAs during cpr-exec
>   migration: only-migratable-modes
>
>  accel/xen/xen-all.c            |   5 +
>  backends/hostmem-epc.c         |  12 +-
>  hmp-commands.hx                |   2 +-
>  hw/core/machine.c              |  22 +++
>  hw/core/qdev.c                 |   1 +
>  hw/intc/apic_common.c          |   2 +-
>  hw/vfio/migration.c            |   3 +-
>  include/exec/cpu-common.h      |   3 +-
>  include/exec/memory.h          |  15 ++
>  include/exec/ramblock.h        |  10 +-
>  include/hw/boards.h            |   1 +
>  include/migration/blocker.h    |   7 +
>  include/migration/cpr.h        |  14 ++
>  include/migration/misc.h       |  11 ++
>  include/migration/vmstate.h    | 133 +++++++++++++++-
>  include/qemu/osdep.h           |   9 ++
>  include/sysemu/runstate.h      |   3 +
>  include/sysemu/seccomp.h       |   1 +
>  include/sysemu/sysemu.h        |   1 -
>  migration/cpr.c                | 131 ++++++++++++++++
>  migration/meson.build          |   3 +
>  migration/migration-hmp-cmds.c |  50 +++++-
>  migration/migration.c          |  48 +++++-
>  migration/migration.h          |   5 +-
>  migration/options.c            |  13 ++
>  migration/precreate.c          | 139 +++++++++++++++++
>  migration/ram.c                |  16 +-
>  migration/savevm.c             | 306 +++++++++++++++++++++++++++++-------
>  migration/savevm.h             |   3 +
>  migration/trace-events         |   7 +
>  migration/vmstate-factory.c    |  78 ++++++++++
>  migration/vmstate-types.c      |  24 +++
>  migration/vmstate.c            |   3 +-
>  qapi/migration.json            |  48 +++++-
>  qemu-options.hx                |  22 ++-
>  replay/replay.c                |   6 +
>  stubs/migr-blocker.c           |   5 +
>  stubs/vmstate.c                |  13 ++
>  system/globals.c               |   1 -
>  system/memory.c                |  19 ++-
>  system/physmem.c               | 346 +++++++++++++++++++++++++++--------------
>  system/qemu-seccomp.c          |  10 +-
>  system/runstate.c              |  29 ++++
>  system/trace-events            |   4 +
>  system/vl.c                    |  26 +++-
>  target/s390x/cpu_models.c      |   4 +-
>  util/oslib-posix.c             |   9 ++
>  util/oslib-win32.c             |   4 +
>  48 files changed, 1417 insertions(+), 210 deletions(-)
>  create mode 100644 include/migration/cpr.h
>  create mode 100644 migration/cpr.c
>  create mode 100644 migration/precreate.c
>  create mode 100644 migration/vmstate-factory.c

Hi Steve,

make check is failing. I applied the series on top of master @
70581940ca (Merge tag 'pull-tcg-20240523' of
https://gitlab.com/rth7680/qemu into staging, 2024-05-23).

$ QTEST_QEMU_BINARY=./qemu-system-x86_64 ./tests/qtest/ivshmem-test
...
qemu-system-x86_64: ../system/physmem.c:1634: qemu_ram_verify_idstr:
Assertion `!strcmp(new_block->idstr, idstr)' failed.

$ QTEST_QEMU_BINARY=./qemu-system-x86_64 \
./tests/qtest/test-x86-cpuid-compat -p \
/x86_64/x86/cpuid/auto-level/pc-2.7
...
qemu-system-x86_64: ../system/physmem.c:1634: qemu_ram_verify_idstr:
Assertion `!strcmp(new_block->idstr, idstr)' failed.

$ QTEST_QEMU_BINARY=./qemu-system-x86_64 ./tests/qtest/qmp-cmd-test -p \
/x86_64/qmp/object-add-failure-modes
...
savevm_state_handler_insert: Detected duplicate SaveStateEntry:
id=ram1/RAMBlock, instance_id=0x0

On 5/24/2024 9:02 AM, Fabiano Rosas wrote:
> Steve Sistare <steven.sistare@oracle.com> writes:
> 
>> This patch series adds the live migration cpr-exec mode.  In this mode, QEMU
>> stops the VM, writes VM state to the migration URI, and directly exec's a
>> new version of QEMU on the same host, replacing the original process while
>> retaining its PID.  Guest RAM is preserved in place, albeit with new virtual
>> addresses.  The user completes the migration by specifying the -incoming
>> option, and by issuing the migrate-incoming command if necessary.  This
>> saves and restores VM state, with minimal guest pause time, so that QEMU may
>> be updated to a new version in between.
>>
>> The new interfaces are:
>>    * cpr-exec (MigMode migration parameter)
>>    * cpr-exec-args (migration parameter)
>>    * memfd-alloc=on (command-line option for -machine)
>>    * only-migratable-modes (command-line argument)
>>
>> The caller sets the mode parameter before invoking the migrate command.
>>
>> Arguments for the new QEMU process are taken from the cpr-exec-args parameter.
>> The first argument should be the path of a new QEMU binary, or a prefix
>> command that exec's the new QEMU binary, and the arguments should include
>> the -incoming option.
>>
>> Memory backend objects must have the share=on attribute, and must be mmap'able
>> in the new QEMU process.  For example, memory-backend-file is acceptable,
>> but memory-backend-ram is not.
>>
>> QEMU must be started with the '-machine memfd-alloc=on' option.  This causes
>> implicit RAM blocks (those not explicitly described by a memory-backend
>> object) to be allocated by mmap'ing a memfd.  Examples include VGA, ROM,
>> and even guest RAM when it is specified without without reference to a
>> memory-backend object.   The memfds are kept open across exec, their values
>> are saved in vmstate which is retrieved after exec, and they are re-mmap'd.
>>
>> The '-only-migratable-modes cpr-exec' option guarantees that the
>> configuration supports cpr-exec.  QEMU will exit at start time if not.
>>
>> Example:
>>
>> In this example, we simply restart the same version of QEMU, but in
>> a real scenario one would set a new QEMU binary path in cpr-exec-args.
>>
>>    # qemu-kvm -monitor stdio -object
>>    memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/ram0,share=on
>>    -m 4G -machine memfd-alloc=on ...
>>
>>    QEMU 9.1.50 monitor - type 'help' for more information
>>    (qemu) info status
>>    VM status: running
>>    (qemu) migrate_set_parameter mode cpr-exec
>>    (qemu) migrate_set_parameter cpr-exec-args qemu-kvm ... -incoming file:vm.state
>>    (qemu) migrate -d file:vm.state
>>    (qemu) QEMU 9.1.50 monitor - type 'help' for more information
>>    (qemu) info status
>>    VM status: running
>>
>> cpr-exec mode preserves attributes of outgoing devices that must be known
>> before the device is created on the incoming side, such as the memfd descriptor
>> number, but currently the migration stream is read after all devices are
>> created.  To solve this problem, I add two VMStateDescription options:
>> precreate and factory.  precreate objects are saved to their own migration
>> stream, distinct from the main stream, and are read early by incoming QEMU,
>> before devices are created.  Factory objects are allocated on demand, without
>> relying on a pre-registered object's opaque address, which is necessary
>> because the devices to which the state will apply have not been created yet
>> and hence have not registered an opaque address to receive the state.
>>
>> This patch series implements a minimal version of cpr-exec.  Future series
>> will add support for:
>>    * vfio
>>    * chardev's without loss of connectivity
>>    * vhost
>>    * fine-grained seccomp controls
>>    * hostmem-memfd
>>    * cpr-exec migration test
>>
>>
>> Steve Sistare (26):
>>    oslib: qemu_clear_cloexec
>>    vl: helper to request re-exec
>>    migration: SAVEVM_FOREACH
>>    migration: delete unused parameter mis
>>    migration: precreate vmstate
>>    migration: precreate vmstate for exec
>>    migration: VMStateId
>>    migration: vmstate_info_void_ptr
>>    migration: vmstate_register_named
>>    migration: vmstate_unregister_named
>>    migration: vmstate_register at init time
>>    migration: vmstate factory object
>>    physmem: ram_block_create
>>    physmem: hoist guest_memfd creation
>>    physmem: hoist host memory allocation
>>    physmem: set ram block idstr earlier
>>    machine: memfd-alloc option
>>    migration: cpr-exec-args parameter
>>    physmem: preserve ram blocks for cpr
>>    migration: cpr-exec mode
>>    migration: migrate_add_blocker_mode
>>    migration: ram block cpr-exec blockers
>>    migration: misc cpr-exec blockers
>>    seccomp: cpr-exec blocker
>>    migration: fix mismatched GPAs during cpr-exec
>>    migration: only-migratable-modes
>>
>>   accel/xen/xen-all.c            |   5 +
>>   backends/hostmem-epc.c         |  12 +-
>>   hmp-commands.hx                |   2 +-
>>   hw/core/machine.c              |  22 +++
>>   hw/core/qdev.c                 |   1 +
>>   hw/intc/apic_common.c          |   2 +-
>>   hw/vfio/migration.c            |   3 +-
>>   include/exec/cpu-common.h      |   3 +-
>>   include/exec/memory.h          |  15 ++
>>   include/exec/ramblock.h        |  10 +-
>>   include/hw/boards.h            |   1 +
>>   include/migration/blocker.h    |   7 +
>>   include/migration/cpr.h        |  14 ++
>>   include/migration/misc.h       |  11 ++
>>   include/migration/vmstate.h    | 133 +++++++++++++++-
>>   include/qemu/osdep.h           |   9 ++
>>   include/sysemu/runstate.h      |   3 +
>>   include/sysemu/seccomp.h       |   1 +
>>   include/sysemu/sysemu.h        |   1 -
>>   migration/cpr.c                | 131 ++++++++++++++++
>>   migration/meson.build          |   3 +
>>   migration/migration-hmp-cmds.c |  50 +++++-
>>   migration/migration.c          |  48 +++++-
>>   migration/migration.h          |   5 +-
>>   migration/options.c            |  13 ++
>>   migration/precreate.c          | 139 +++++++++++++++++
>>   migration/ram.c                |  16 +-
>>   migration/savevm.c             | 306 +++++++++++++++++++++++++++++-------
>>   migration/savevm.h             |   3 +
>>   migration/trace-events         |   7 +
>>   migration/vmstate-factory.c    |  78 ++++++++++
>>   migration/vmstate-types.c      |  24 +++
>>   migration/vmstate.c            |   3 +-
>>   qapi/migration.json            |  48 +++++-
>>   qemu-options.hx                |  22 ++-
>>   replay/replay.c                |   6 +
>>   stubs/migr-blocker.c           |   5 +
>>   stubs/vmstate.c                |  13 ++
>>   system/globals.c               |   1 -
>>   system/memory.c                |  19 ++-
>>   system/physmem.c               | 346 +++++++++++++++++++++++++++--------------
>>   system/qemu-seccomp.c          |  10 +-
>>   system/runstate.c              |  29 ++++
>>   system/trace-events            |   4 +
>>   system/vl.c                    |  26 +++-
>>   target/s390x/cpu_models.c      |   4 +-
>>   util/oslib-posix.c             |   9 ++
>>   util/oslib-win32.c             |   4 +
>>   48 files changed, 1417 insertions(+), 210 deletions(-)
>>   create mode 100644 include/migration/cpr.h
>>   create mode 100644 migration/cpr.c
>>   create mode 100644 migration/precreate.c
>>   create mode 100644 migration/vmstate-factory.c
> 
> Hi Steve,
> 
> make check is failing. I applied the series on top of master @
> 70581940ca (Merge tag 'pull-tcg-20240523' of
> https://gitlab.com/rth7680/qemu into staging, 2024-05-23).
> 
> $ QTEST_QEMU_BINARY=./qemu-system-x86_64 ./tests/qtest/ivshmem-test
> ...
> qemu-system-x86_64: ../system/physmem.c:1634: qemu_ram_verify_idstr:
> Assertion `!strcmp(new_block->idstr, idstr)' failed.
> 
> $ QTEST_QEMU_BINARY=./qemu-system-x86_64 \
> ./tests/qtest/test-x86-cpuid-compat -p \
> /x86_64/x86/cpuid/auto-level/pc-2.7
> ...
> qemu-system-x86_64: ../system/physmem.c:1634: qemu_ram_verify_idstr:
> Assertion `!strcmp(new_block->idstr, idstr)' failed.
> 
> $ QTEST_QEMU_BINARY=./qemu-system-x86_64 ./tests/qtest/qmp-cmd-test -p \
> /x86_64/qmp/object-add-failure-modes
> ...
> savevm_state_handler_insert: Detected duplicate SaveStateEntry:
> id=ram1/RAMBlock, instance_id=0x0

Thank you very much, I will investigate.

I suspect the vmstate dup error is due to this bug which I hit after
posting the patches:
-------------------------------
diff --git a/migration/savevm.c b/migration/savevm.c
index bb7fd9f..54aa233 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1012,7 +1012,7 @@ void vmstate_unregister_named(const char *vmsd_name,
      SaveStateEntry *se, *new_se;
      VMStateId idstr;

-    snprintf(idstr, sizeof(idstr), "%s/%s", vmsd_name, instance_name);
+    snprintf(idstr, sizeof(idstr), "%s/%s", instance_name, vmsd_name);

      SAVEVM_FOREACH_SAFE_ALL(se, entry, new_se) {
          if (!strcmp(se->idstr, idstr) &&
-----------------------------------

- Steve

Hi Peter, Hi Fabiano,
   Will you have time to review the migration guts of this series any time soon?
In particular:

[PATCH V1 05/26] migration: precreate vmstate
[PATCH V1 06/26] migration: precreate vmstate for exec
[PATCH V1 12/26] migration: vmstate factory object
[PATCH V1 18/26] migration: cpr-exec-args parameter
[PATCH V1 20/26] migration: cpr-exec mode

- Steve

On 4/29/2024 11:55 AM, Steve Sistare wrote:
> This patch series adds the live migration cpr-exec mode.  In this mode, QEMU
> stops the VM, writes VM state to the migration URI, and directly exec's a
> new version of QEMU on the same host, replacing the original process while
> retaining its PID.  Guest RAM is preserved in place, albeit with new virtual
> addresses.  The user completes the migration by specifying the -incoming
> option, and by issuing the migrate-incoming command if necessary.  This
> saves and restores VM state, with minimal guest pause time, so that QEMU may
> be updated to a new version in between.
> 
> The new interfaces are:
>    * cpr-exec (MigMode migration parameter)
>    * cpr-exec-args (migration parameter)
>    * memfd-alloc=on (command-line option for -machine)
>    * only-migratable-modes (command-line argument)
> 
> The caller sets the mode parameter before invoking the migrate command.
> 
> Arguments for the new QEMU process are taken from the cpr-exec-args parameter.
> The first argument should be the path of a new QEMU binary, or a prefix
> command that exec's the new QEMU binary, and the arguments should include
> the -incoming option.
> 
> Memory backend objects must have the share=on attribute, and must be mmap'able
> in the new QEMU process.  For example, memory-backend-file is acceptable,
> but memory-backend-ram is not.
> 
> QEMU must be started with the '-machine memfd-alloc=on' option.  This causes
> implicit RAM blocks (those not explicitly described by a memory-backend
> object) to be allocated by mmap'ing a memfd.  Examples include VGA, ROM,
> and even guest RAM when it is specified without without reference to a
> memory-backend object.   The memfds are kept open across exec, their values
> are saved in vmstate which is retrieved after exec, and they are re-mmap'd.
> 
> The '-only-migratable-modes cpr-exec' option guarantees that the
> configuration supports cpr-exec.  QEMU will exit at start time if not.
> 
> Example:
> 
> In this example, we simply restart the same version of QEMU, but in
> a real scenario one would set a new QEMU binary path in cpr-exec-args.
> 
>    # qemu-kvm -monitor stdio -object
>    memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/ram0,share=on
>    -m 4G -machine memfd-alloc=on ...
> 
>    QEMU 9.1.50 monitor - type 'help' for more information
>    (qemu) info status
>    VM status: running
>    (qemu) migrate_set_parameter mode cpr-exec
>    (qemu) migrate_set_parameter cpr-exec-args qemu-kvm ... -incoming file:vm.state
>    (qemu) migrate -d file:vm.state
>    (qemu) QEMU 9.1.50 monitor - type 'help' for more information
>    (qemu) info status
>    VM status: running
> 
> cpr-exec mode preserves attributes of outgoing devices that must be known
> before the device is created on the incoming side, such as the memfd descriptor
> number, but currently the migration stream is read after all devices are
> created.  To solve this problem, I add two VMStateDescription options:
> precreate and factory.  precreate objects are saved to their own migration
> stream, distinct from the main stream, and are read early by incoming QEMU,
> before devices are created.  Factory objects are allocated on demand, without
> relying on a pre-registered object's opaque address, which is necessary
> because the devices to which the state will apply have not been created yet
> and hence have not registered an opaque address to receive the state.
> 
> This patch series implements a minimal version of cpr-exec.  Future series
> will add support for:
>    * vfio
>    * chardev's without loss of connectivity
>    * vhost
>    * fine-grained seccomp controls
>    * hostmem-memfd
>    * cpr-exec migration test
> 
> 
> Steve Sistare (26):
>    oslib: qemu_clear_cloexec
>    vl: helper to request re-exec
>    migration: SAVEVM_FOREACH
>    migration: delete unused parameter mis
>    migration: precreate vmstate
>    migration: precreate vmstate for exec
>    migration: VMStateId
>    migration: vmstate_info_void_ptr
>    migration: vmstate_register_named
>    migration: vmstate_unregister_named
>    migration: vmstate_register at init time
>    migration: vmstate factory object
>    physmem: ram_block_create
>    physmem: hoist guest_memfd creation
>    physmem: hoist host memory allocation
>    physmem: set ram block idstr earlier
>    machine: memfd-alloc option
>    migration: cpr-exec-args parameter
>    physmem: preserve ram blocks for cpr
>    migration: cpr-exec mode
>    migration: migrate_add_blocker_mode
>    migration: ram block cpr-exec blockers
>    migration: misc cpr-exec blockers
>    seccomp: cpr-exec blocker
>    migration: fix mismatched GPAs during cpr-exec
>    migration: only-migratable-modes
> 
>   accel/xen/xen-all.c            |   5 +
>   backends/hostmem-epc.c         |  12 +-
>   hmp-commands.hx                |   2 +-
>   hw/core/machine.c              |  22 +++
>   hw/core/qdev.c                 |   1 +
>   hw/intc/apic_common.c          |   2 +-
>   hw/vfio/migration.c            |   3 +-
>   include/exec/cpu-common.h      |   3 +-
>   include/exec/memory.h          |  15 ++
>   include/exec/ramblock.h        |  10 +-
>   include/hw/boards.h            |   1 +
>   include/migration/blocker.h    |   7 +
>   include/migration/cpr.h        |  14 ++
>   include/migration/misc.h       |  11 ++
>   include/migration/vmstate.h    | 133 +++++++++++++++-
>   include/qemu/osdep.h           |   9 ++
>   include/sysemu/runstate.h      |   3 +
>   include/sysemu/seccomp.h       |   1 +
>   include/sysemu/sysemu.h        |   1 -
>   migration/cpr.c                | 131 ++++++++++++++++
>   migration/meson.build          |   3 +
>   migration/migration-hmp-cmds.c |  50 +++++-
>   migration/migration.c          |  48 +++++-
>   migration/migration.h          |   5 +-
>   migration/options.c            |  13 ++
>   migration/precreate.c          | 139 +++++++++++++++++
>   migration/ram.c                |  16 +-
>   migration/savevm.c             | 306 +++++++++++++++++++++++++++++-------
>   migration/savevm.h             |   3 +
>   migration/trace-events         |   7 +
>   migration/vmstate-factory.c    |  78 ++++++++++
>   migration/vmstate-types.c      |  24 +++
>   migration/vmstate.c            |   3 +-
>   qapi/migration.json            |  48 +++++-
>   qemu-options.hx                |  22 ++-
>   replay/replay.c                |   6 +
>   stubs/migr-blocker.c           |   5 +
>   stubs/vmstate.c                |  13 ++
>   system/globals.c               |   1 -
>   system/memory.c                |  19 ++-
>   system/physmem.c               | 346 +++++++++++++++++++++++++++--------------
>   system/qemu-seccomp.c          |  10 +-
>   system/runstate.c              |  29 ++++
>   system/trace-events            |   4 +
>   system/vl.c                    |  26 +++-
>   target/s390x/cpu_models.c      |   4 +-
>   util/oslib-posix.c             |   9 ++
>   util/oslib-win32.c             |   4 +
>   48 files changed, 1417 insertions(+), 210 deletions(-)
>   create mode 100644 include/migration/cpr.h
>   create mode 100644 migration/cpr.c
>   create mode 100644 migration/precreate.c
>   create mode 100644 migration/vmstate-factory.c
>

Steven Sistare <steven.sistare@oracle.com> writes:

> Hi Peter, Hi Fabiano,
>    Will you have time to review the migration guts of this series any time soon?
> In particular:
>
> [PATCH V1 05/26] migration: precreate vmstate
> [PATCH V1 06/26] migration: precreate vmstate for exec
> [PATCH V1 12/26] migration: vmstate factory object
> [PATCH V1 18/26] migration: cpr-exec-args parameter
> [PATCH V1 20/26] migration: cpr-exec mode
>

I'll get to them this week. I'm trying to make some progress with my own
code before I forget how to program. I'm also trying to find some time
to implement the device options in the migration tests so we can stop
these virtio-* breakages that have been popping up.

Conference back then pto until today, so tomorrow will be my first working
day after those. Sorry Steve, will try my best to read it before next week.
I didn't dare to read too much my inbox yet.  A bit scared but need to face
it tomorrow.

On Mon, May 20, 2024, 6:28 p.m. Fabiano Rosas <farosas@suse.de> wrote:

> Steven Sistare <steven.sistare@oracle.com> writes:
>
> > Hi Peter, Hi Fabiano,
> >    Will you have time to review the migration guts of this series any
> time soon?
> > In particular:
> >
> > [PATCH V1 05/26] migration: precreate vmstate
> > [PATCH V1 06/26] migration: precreate vmstate for exec
> > [PATCH V1 12/26] migration: vmstate factory object
> > [PATCH V1 18/26] migration: cpr-exec-args parameter
> > [PATCH V1 20/26] migration: cpr-exec mode
> >
>
> I'll get to them this week. I'm trying to make some progress with my own
> code before I forget how to program. I'm also trying to find some time
> to implement the device options in the migration tests so we can stop
> these virtio-* breakages that have been popping up.
>
>

I understand, thanks.  If I can help with any of your todo list,
just ask - steve

On 5/20/2024 10:31 PM, Peter Xu wrote:
> Conference back then pto until today, so tomorrow will be my first working day 
> after those. Sorry Steve, will try my best to read it before next week. I didn't 
> dare to read too much my inbox yet.  A bit scared but need to face it tomorrow.
> 
> On Mon, May 20, 2024, 6:28 p.m. Fabiano Rosas <farosas@suse.de 
> <mailto:farosas@suse.de>> wrote:
> 
>     Steven Sistare <steven.sistare@oracle.com
>     <mailto:steven.sistare@oracle.com>> writes:
> 
>      > Hi Peter, Hi Fabiano,
>      >    Will you have time to review the migration guts of this series any
>     time soon?
>      > In particular:
>      >
>      > [PATCH V1 05/26] migration: precreate vmstate
>      > [PATCH V1 06/26] migration: precreate vmstate for exec
>      > [PATCH V1 12/26] migration: vmstate factory object
>      > [PATCH V1 18/26] migration: cpr-exec-args parameter
>      > [PATCH V1 20/26] migration: cpr-exec mode
>      >
> 
>     I'll get to them this week. I'm trying to make some progress with my own
>     code before I forget how to program. I'm also trying to find some time
>     to implement the device options in the migration tests so we can stop
>     these virtio-* breakages that have been popping up.
> 

On Tue, May 21, 2024 at 07:46:12AM -0400, Steven Sistare wrote:
> I understand, thanks.  If I can help with any of your todo list,
> just ask - steve

Thanks for offering the help, Steve.  Started looking at this today, then I
found that I miss something high-level.  Let me ask here, and let me
apologize already for starting to throw multiple questions..

IIUC the whole idea of this patchset is to allow efficient QEMU upgrade, in
this case not host kernel but QEMU-only, and/or upper.

Is there any justification on why the complexity is needed here?  It looks
to me this one is more involved than cpr-reboot, so I'm thinking how much
we can get from the complexity, and whether it's worthwhile.  1000+ LOC is
the min support, and if we even expect more to come, that's really
important, IMHO.

For example, what's the major motivation of this whole work?  Is that more
on performance, or is it more for supporting the special devices like VFIO
which we used to not support, or something else?  I can't find them in
whatever cover letter I can find, including this one.

Firstly, regarding performance, IMHO it'll be always nice to share even
some very fundamental downtime measurement comparisons using the new exec
mode v.s. the old migration ways to upgrade QEMU binary.  Do you perhaps
have some number on hand when you started working on this feature years
ago?  Or maybe some old links on the list would help too, as I didn't
follow this work since the start.

On VFIO, IIUC you started out this project without VFIO migration being
there.  Now we have VFIO migration so not sure how much it would work for
the upgrade use case. Even with current VFIO migration, we may not want to
migrate device states for a local upgrade I suppose, as that can be a lot
depending on the type of device assigned.  However it'll be nice to discuss
this too if this is the major purpose of the series.

I think one other challenge on QEMU upgrade with VFIO devices is that the
dest QEMU won't be able to open the VFIO device when the src QEMU is still
using it as the owner.  IIUC this is a similar condition where QEMU wants
to have proper ownership transfer of a shared block device, and AFAIR right
now we resolved that issue using some form of file lock on the image file.
In this case it won't easily apply to a VFIO dev fd, but maybe we still
have other approaches, not sure whether you investigated any.  E.g. could
the VFIO handle be passed over using unix scm rights?  I think this might
remove one dependency of using exec which can cause quite some difference
v.s. a generic migration (from which regard, cpr-reboot is still a pretty
generic migration).

You also mentioned vhost/tap, is that also a major goal of this series in
the follow up patchsets?  Is this a problem only because this solution will
do exec?  Can it work if either the exec()ed qemu or dst qemu create the
vhost/tap fds when boot?

Meanwhile, could you elaborate a bit on the implication on chardevs?  From
what I read in the doc update it looks like a major part of work in the
future, but I don't yet understand the issue..  Is it also relevant to the
exec() approach?

In all cases, some of such discussion would be really appreciated.  And if
you used to consider other approaches to solve this problem it'll be great
to mention how you chose this way.  Considering this work contains too many
things, it'll be nice if such discussion can start with the fundamentals,
e.g. on why exec() is a must.

Thanks,

-- 
Peter Xu

On 5/27/2024 1:45 PM, Peter Xu wrote:
> On Tue, May 21, 2024 at 07:46:12AM -0400, Steven Sistare wrote:
>> I understand, thanks.  If I can help with any of your todo list,
>> just ask - steve
> 
> Thanks for offering the help, Steve.  Started looking at this today, then I
> found that I miss something high-level.  Let me ask here, and let me
> apologize already for starting to throw multiple questions..
> 
> IIUC the whole idea of this patchset is to allow efficient QEMU upgrade, in
> this case not host kernel but QEMU-only, and/or upper.
> 
> Is there any justification on why the complexity is needed here?  It looks
> to me this one is more involved than cpr-reboot, so I'm thinking how much
> we can get from the complexity, and whether it's worthwhile.  1000+ LOC is
> the min support, and if we even expect more to come, that's really
> important, IMHO.
> 
> For example, what's the major motivation of this whole work?  Is that more
> on performance, or is it more for supporting the special devices like VFIO
> which we used to not support, or something else?  I can't find them in
> whatever cover letter I can find, including this one.
> 
> Firstly, regarding performance, IMHO it'll be always nice to share even
> some very fundamental downtime measurement comparisons using the new exec
> mode v.s. the old migration ways to upgrade QEMU binary.  Do you perhaps
> have some number on hand when you started working on this feature years
> ago?  Or maybe some old links on the list would help too, as I didn't
> follow this work since the start.
> 
> On VFIO, IIUC you started out this project without VFIO migration being
> there.  Now we have VFIO migration so not sure how much it would work for
> the upgrade use case. Even with current VFIO migration, we may not want to
> migrate device states for a local upgrade I suppose, as that can be a lot
> depending on the type of device assigned.  However it'll be nice to discuss
> this too if this is the major purpose of the series.
> 
> I think one other challenge on QEMU upgrade with VFIO devices is that the
> dest QEMU won't be able to open the VFIO device when the src QEMU is still
> using it as the owner.  IIUC this is a similar condition where QEMU wants
> to have proper ownership transfer of a shared block device, and AFAIR right
> now we resolved that issue using some form of file lock on the image file.
> In this case it won't easily apply to a VFIO dev fd, but maybe we still
> have other approaches, not sure whether you investigated any.  E.g. could
> the VFIO handle be passed over using unix scm rights?  I think this might
> remove one dependency of using exec which can cause quite some difference
> v.s. a generic migration (from which regard, cpr-reboot is still a pretty
> generic migration).
> 
> You also mentioned vhost/tap, is that also a major goal of this series in
> the follow up patchsets?  Is this a problem only because this solution will
> do exec?  Can it work if either the exec()ed qemu or dst qemu create the
> vhost/tap fds when boot?
> 
> Meanwhile, could you elaborate a bit on the implication on chardevs?  From
> what I read in the doc update it looks like a major part of work in the
> future, but I don't yet understand the issue..  Is it also relevant to the
> exec() approach?
> 
> In all cases, some of such discussion would be really appreciated.  And if
> you used to consider other approaches to solve this problem it'll be great
> to mention how you chose this way.  Considering this work contains too many
> things, it'll be nice if such discussion can start with the fundamentals,
> e.g. on why exec() is a must.

The main goal of cpr-exec is providing a fast and reliable way to update
qemu. cpr-reboot is not fast enough or general enough.  It requires the
guest to support suspend and resume for all devices, and that takes seconds.
If one actually reboots the host, that adds more seconds, depending on
system services.  cpr-exec takes 0.1 secs, and works every time, unlike
like migration which can fail to converge on a busy system.  Live migration
also consumes more system and network resources.  cpr-exec seamlessly
preserves client connections by preserving chardevs, and overall provides
a much nicer user experience.

chardev's are preserved by keeping their fd open across the exec, and
remembering the value of the fd in precreate vmstate so that new qemu
can associate the fd with the chardev rather than opening a new one.

The approach of preserving open file descriptors is very general and applicable
to all kinds of devices, regardless of whether they support live migration
in hardware.  Device fd's are preserved using the same mechanism as for
chardevs.

Devices that support live migration in hardware do not like to live migrate
in place to the same node.  It is not what they are designed for, and some
implementations will flat out fail because the source and target interfaces
are the same.

For vhost/tap, sometimes the management layer opens the dev and passes an
fd to qemu, and sometimes qemu opens the dev.  The upcoming vhost/tap support
allows both.  For the case where qemu opens the dev, the fd is preserved
using the same mechanism as for chardevs.

The fundamental requirements of this work are:
   - precreate vmstate
   - preserve open file descriptors

Direct exec from old to new qemu is not a hard requirement.   However,
it is simple, with few complications, and works with Oracle's cloud
containers, so it is the method I am most interested in finishing first.

I believe everything could also be made to work by using SCM_RIGHTS to
send fd's to a new qemu process that is started by some external means.
It would be requested with MIG_MODE_CPR_SCM (or some better name), and
would co-exist with MIG_MODE_CPR_EXEC.

- Steve

On 4/29/2024 11:55 AM, Steve Sistare wrote:
> This patch series adds the live migration cpr-exec mode.

Here is the text I plan to add to docs/devel/migration/CPR.rst.  It is
premature for me to submit this as a patch, because it includes all
the functionality I plan to add in this and future series, but it may
help you while reviewing this series.

- Steve

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

cpr-exec mode
---------------

In this mode, QEMU stops the VM, writes VM state to the migration
URI, and directly exec's a new version of QEMU on the same host,
replacing the original process while retaining its PID.  Guest RAM is
preserved in place, albeit with new virtual addresses.  The user
completes the migration by specifying the ``-incoming`` option, and
by issuing the ``migrate-incoming`` command if necessary; see details
below.

This mode supports vfio devices by preserving device descriptors and
hence kernel state across the exec, even for devices that do not
support live migration, and preserves tap and vhost descriptors.

cpr-exec also preserves descriptors for a subset of chardevs,
including socket, file, parallel, pipe, serial, pty, stdio, and null.
chardevs that support cpr-exec have the QEMU_CHAR_FEATURE_CPR set in
the Chardev object.  The client side of a preserved chardev sees no
loss of connectivity during cpr-exec.  More chardevs could be
preserved with additional developement.

All chardevs have a ``reopen-on-cpr`` option which causes the chardev
to be closed and reopened during cpr-exec.  This can be set to allow
cpr-exec when the configuration includes a chardev (such as vc) that
does not have QEMU_CHAR_FEATURE_CPR.

Because the old and new QEMU instances are not active concurrently,
the URI cannot be a type that streams data from one instance to the
other.

Usage
^^^^^

Arguments for the new QEMU process are taken from the
@cpr-exec-args parameter.  The first argument should be the
path of a new QEMU binary, or a prefix command that exec's the
new QEMU binary, and the arguments should include the ''-incoming''
option.

Memory backend objects must have the ``share=on`` attribute, and
must be mmap'able in the new QEMU process.  For example,
memory-backend-file is acceptable, but memory-backend-ram is
not.

The VM must be started with the ``-machine memfd-alloc=on``
option.  This causes implicit RAM blocks (those not explicitly
described by a memory-backend object) to be allocated by
mmap'ing a memfd.  Examples include VGA, ROM, and even guest
RAM when it is specified without without reference to a
memory-backend object.

Add the ``-only-migratable-modes cpr-exec`` option to guarantee that
the configuration supports cpr-exec.  QEMU will exit at start time
if not.

Outgoing:
   * Set the migration mode parameter to ``cpr-exec``.
   * Set the ``cpr-exec-args`` parameter.
   * Issue the ``migrate`` command.  It is recommended the the URI be
     a ``file`` type, but one can use other types such as ``exec``,
     provided the command captures all the data from the outgoing side,
     and provides all the data to the incoming side.

Incoming:
   * You do not need to explicitly start new QEMU.  It is started as
     a side effect of the migrate command above.
   * If the VM was running when the outgoing ``migrate`` command was
     issued, then QEMU automatically resumes VM execution.

Example 1: incoming URI
^^^^^^^^^^^^^^^^^^^^^^^

In these examples, we simply restart the same version of QEMU, but in
a real scenario one would set a new QEMU binary path in cpr-exec-args.

::

   # qemu-kvm -monitor stdio
   -object 
memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/ram0,share=on -m 4G
   -machine memfd-alloc=on
   ...

   QEMU 9.1.50 monitor - type 'help' for more information
   (qemu) info status
   VM status: running
   (qemu) migrate_set_parameter mode cpr-exec
   (qemu) migrate_set_parameter cpr-exec-args qemu-kvm ... -incoming 
file:vm.state
   (qemu) migrate -d file:vm.state
   (qemu) QEMU 9.1.50 monitor - type 'help' for more information
   (qemu) info status
   VM status: running

Example 2: incoming defer
^^^^^^^^^^^^^^^^^^^^^^^^^
::

   # qemu-kvm -monitor stdio
   -object 
memory-backend-file,id=ram0,size=4G,mem-path=/dev/shm/ram0,share=on -m 4G
   -machine memfd-alloc=on
   ...

   QEMU 9.1.50 monitor - type 'help' for more information
   (qemu) info status
   VM status: running
   (qemu) migrate_set_parameter mode cpr-exec
   (qemu) migrate_set_parameter cpr-exec-args qemu-kvm ... -incoming defer
   (qemu) migrate -d file:vm.state
   (qemu) QEMU 9.1.50 monitor - type 'help' for more information
   (qemu) info status
   status: paused (inmigrate)
   (qemu) migrate_incoming file:vm.state
   (qemu) info status
   VM status: running


Caveats
^^^^^^^

cpr-exec mode may not be used with postcopy, background-snapshot,
or COLO.

cpr-exec mode requires permission to use the exec system call, which
is denied by certain sandbox options, such as spawn.  Use finer
grained controls to allow exec, eg:
``-sandbox on,fork=deny,ns=deny,exec=allow``

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

On Thu, May 02, 2024 at 12:13:17PM -0400, Steven Sistare wrote:
> On 4/29/2024 11:55 AM, Steve Sistare wrote:
> > This patch series adds the live migration cpr-exec mode.
> 
> Here is the text I plan to add to docs/devel/migration/CPR.rst.  It is
> premature for me to submit this as a patch, because it includes all
> the functionality I plan to add in this and future series, but it may
> help you while reviewing this series.

I haven't reached this series at all yet but thanks for sending this,
definitely helpful for reviews.  I almost tried to ask for it. :)

I don't think it's an issue to send doc updates without full
implementations ready.  We can still mark things as BTD even in doc IMHO,
and it may help to provide a better picture of the whole thing if e.g. this
series only implemented part of them, to either reviewers or users (for the
latter, if the partially impl feature can already be consumed).

Thanks,

-- 
Peter Xu