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[PATCH v2] hw/arm/virt: Avoid unexpected warning from Linux guest on host with Fujitsu CPUs

Zhenyu Zhang posted 1 patch 11 months ago
There is a newer version of this series
hw/arm/virt.c | 1 +
1 file changed, 1 insertion(+)
[PATCH v2] hw/arm/virt: Avoid unexpected warning from Linux guest on host with Fujitsu CPUs
Posted by Zhenyu Zhang 11 months ago 
Multiple warning messages and corresponding backtraces are observed when Linux
guest is booted on the host with Fujitsu CPUs. One of them is shown as below.

[    0.032443] ------------[ cut here ]------------
[    0.032446] uart-pl011 9000000.pl011: ARCH_DMA_MINALIGN smaller than
CTR_EL0.CWG (128 < 256)
[    0.032454] WARNING: CPU: 0 PID: 1 at arch/arm64/mm/dma-mapping.c:54
arch_setup_dma_ops+0xbc/0xcc
[    0.032470] Modules linked in:
[    0.032475] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.14.0-452.el9.aarch64
[    0.032481] Hardware name: linux,dummy-virt (DT)
[    0.032484] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[    0.032490] pc : arch_setup_dma_ops+0xbc/0xcc
[    0.032496] lr : arch_setup_dma_ops+0xbc/0xcc
[    0.032501] sp : ffff80008003b860
[    0.032503] x29: ffff80008003b860 x28: 0000000000000000 x27: ffffaae4b949049c
[    0.032510] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
[    0.032517] x23: 0000000000000100 x22: 0000000000000000 x21: 0000000000000000
[    0.032523] x20: 0000000100000000 x19: ffff2f06c02ea400 x18: ffffffffffffffff
[    0.032529] x17: 00000000208a5f76 x16: 000000006589dbcb x15: ffffaae4ba071c89
[    0.032535] x14: 0000000000000000 x13: ffffaae4ba071c84 x12: 455f525443206e61
[    0.032541] x11: 68742072656c6c61 x10: 0000000000000029 x9 : ffffaae4b7d21da4
[    0.032547] x8 : 0000000000000029 x7 : 4c414e494d5f414d x6 : 0000000000000029
[    0.032553] x5 : 000000000000000f x4 : ffffaae4b9617a00 x3 : 0000000000000001
[    0.032558] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff2f06c029be40
[    0.032564] Call trace:
[    0.032566]  arch_setup_dma_ops+0xbc/0xcc
[    0.032572]  of_dma_configure_id+0x138/0x300
[    0.032591]  amba_dma_configure+0x34/0xc0
[    0.032600]  really_probe+0x78/0x3dc
[    0.032614]  __driver_probe_device+0x108/0x160
[    0.032619]  driver_probe_device+0x44/0x114
[    0.032624]  __device_attach_driver+0xb8/0x14c
[    0.032629]  bus_for_each_drv+0x88/0xe4
[    0.032634]  __device_attach+0xb0/0x1e0
[    0.032638]  device_initial_probe+0x18/0x20
[    0.032643]  bus_probe_device+0xa8/0xb0
[    0.032648]  device_add+0x4b4/0x6c0
[    0.032652]  amba_device_try_add.part.0+0x48/0x360
[    0.032657]  amba_device_add+0x104/0x144
[    0.032662]  of_amba_device_create.isra.0+0x100/0x1c4
[    0.032666]  of_platform_bus_create+0x294/0x35c
[    0.032669]  of_platform_populate+0x5c/0x150
[    0.032672]  of_platform_default_populate_init+0xd0/0xec
[    0.032697]  do_one_initcall+0x4c/0x2e0
[    0.032701]  do_initcalls+0x100/0x13c
[    0.032707]  kernel_init_freeable+0x1c8/0x21c
[    0.032712]  kernel_init+0x28/0x140
[    0.032731]  ret_from_fork+0x10/0x20
[    0.032735] ---[ end trace 0000000000000000 ]---

In Linux, a check is applied to every device which is exposed through
device-tree node. The warning message is raised when the device isn't 
DMA coherent and the cache line size is larger than ARCH_DMA_MINALIGN 
(128 bytes). The cache line is sorted from CTR_EL0[CWG], which corresponds 
to 256 bytes on the guest CPUs. The DMA coherent capability is claimed 
through 'dma-coherent' property in their device-tree nodes or parent nodes.

Fix the issue by adding 'dma-coherent' property to the device-tree root 
node, meaning all devices are capable of DMA coherent by default.

Signed-off-by: Zhenyu Zhang <zhenyzha@redhat.com>
---
 hw/arm/virt.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 3c93c0c0a6..0680b792bc 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -270,6 +270,7 @@ static void create_fdt(VirtMachineState *vms)
     qemu_fdt_setprop_cell(fdt, "/", "#address-cells", 0x2);
     qemu_fdt_setprop_cell(fdt, "/", "#size-cells", 0x2);
     qemu_fdt_setprop_string(fdt, "/", "model", "linux,dummy-virt");
+    qemu_fdt_setprop(fdt, "/", "dma-coherent", NULL, 0);
 
     /* /chosen must exist for load_dtb to fill in necessary properties later */
     qemu_fdt_add_subnode(fdt, "/chosen");
-- 
2.43.0
Re: [PATCH v2] hw/arm/virt: Avoid unexpected warning from Linux guest on host with Fujitsu CPUs
Posted by Peter Maydell 11 months ago 
On Tue, 11 Jun 2024 at 13:12, Zhenyu Zhang <zhenyzha@redhat.com> wrote:
>
> Multiple warning messages and corresponding backtraces are observed when Linux
> guest is booted on the host with Fujitsu CPUs. One of them is shown as below.
>
> [    0.032443] ------------[ cut here ]------------
> [    0.032446] uart-pl011 9000000.pl011: ARCH_DMA_MINALIGN smaller than
> CTR_EL0.CWG (128 < 256)
> [    0.032454] WARNING: CPU: 0 PID: 1 at arch/arm64/mm/dma-mapping.c:54
> arch_setup_dma_ops+0xbc/0xcc
> [    0.032470] Modules linked in:
> [    0.032475] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.14.0-452.el9.aarch64
> [    0.032481] Hardware name: linux,dummy-virt (DT)
> [    0.032484] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [    0.032490] pc : arch_setup_dma_ops+0xbc/0xcc
> [    0.032496] lr : arch_setup_dma_ops+0xbc/0xcc
> [    0.032501] sp : ffff80008003b860
> [    0.032503] x29: ffff80008003b860 x28: 0000000000000000 x27: ffffaae4b949049c
> [    0.032510] x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
> [    0.032517] x23: 0000000000000100 x22: 0000000000000000 x21: 0000000000000000
> [    0.032523] x20: 0000000100000000 x19: ffff2f06c02ea400 x18: ffffffffffffffff
> [    0.032529] x17: 00000000208a5f76 x16: 000000006589dbcb x15: ffffaae4ba071c89
> [    0.032535] x14: 0000000000000000 x13: ffffaae4ba071c84 x12: 455f525443206e61
> [    0.032541] x11: 68742072656c6c61 x10: 0000000000000029 x9 : ffffaae4b7d21da4
> [    0.032547] x8 : 0000000000000029 x7 : 4c414e494d5f414d x6 : 0000000000000029
> [    0.032553] x5 : 000000000000000f x4 : ffffaae4b9617a00 x3 : 0000000000000001
> [    0.032558] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff2f06c029be40
> [    0.032564] Call trace:
> [    0.032566]  arch_setup_dma_ops+0xbc/0xcc
> [    0.032572]  of_dma_configure_id+0x138/0x300
> [    0.032591]  amba_dma_configure+0x34/0xc0
> [    0.032600]  really_probe+0x78/0x3dc
> [    0.032614]  __driver_probe_device+0x108/0x160
> [    0.032619]  driver_probe_device+0x44/0x114
> [    0.032624]  __device_attach_driver+0xb8/0x14c
> [    0.032629]  bus_for_each_drv+0x88/0xe4
> [    0.032634]  __device_attach+0xb0/0x1e0
> [    0.032638]  device_initial_probe+0x18/0x20
> [    0.032643]  bus_probe_device+0xa8/0xb0
> [    0.032648]  device_add+0x4b4/0x6c0
> [    0.032652]  amba_device_try_add.part.0+0x48/0x360
> [    0.032657]  amba_device_add+0x104/0x144
> [    0.032662]  of_amba_device_create.isra.0+0x100/0x1c4
> [    0.032666]  of_platform_bus_create+0x294/0x35c
> [    0.032669]  of_platform_populate+0x5c/0x150
> [    0.032672]  of_platform_default_populate_init+0xd0/0xec
> [    0.032697]  do_one_initcall+0x4c/0x2e0
> [    0.032701]  do_initcalls+0x100/0x13c
> [    0.032707]  kernel_init_freeable+0x1c8/0x21c
> [    0.032712]  kernel_init+0x28/0x140
> [    0.032731]  ret_from_fork+0x10/0x20
> [    0.032735] ---[ end trace 0000000000000000 ]---
>
> In Linux, a check is applied to every device which is exposed through
> device-tree node. The warning message is raised when the device isn't
> DMA coherent and the cache line size is larger than ARCH_DMA_MINALIGN
> (128 bytes). The cache line is sorted from CTR_EL0[CWG], which corresponds
> to 256 bytes on the guest CPUs. The DMA coherent capability is claimed
> through 'dma-coherent' property in their device-tree nodes or parent nodes.
>
> Fix the issue by adding 'dma-coherent' property to the device-tree root
> node, meaning all devices are capable of DMA coherent by default.
>
> Signed-off-by: Zhenyu Zhang <zhenyzha@redhat.com>
> ---
>  hw/arm/virt.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 3c93c0c0a6..0680b792bc 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -270,6 +270,7 @@ static void create_fdt(VirtMachineState *vms)
>      qemu_fdt_setprop_cell(fdt, "/", "#address-cells", 0x2);
>      qemu_fdt_setprop_cell(fdt, "/", "#size-cells", 0x2);
>      qemu_fdt_setprop_string(fdt, "/", "model", "linux,dummy-virt");

We should have a comment explaining why we do this:

   /*
    * For QEMU, all DMA is coherent. Advertising this in the root node
    * has two benefits:
    *  - it avoids potential bugs where we forget to mark a DMA
    *    capable device as being dma-coherent
    *  - it avoids spurious warnings from the Linux kernel about
    *    devices which can't do DMA at all
    */

> +    qemu_fdt_setprop(fdt, "/", "dma-coherent", NULL, 0);
>
>      /* /chosen must exist for load_dtb to fill in necessary properties later */
>      qemu_fdt_add_subnode(fdt, "/chosen");

thanks
-- PMM

Patchew 2.1-devel-8c64711

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