Upcoming AMD uarch will support Bus Lock Detect (called Bus Lock Trap
in AMD docs). Bus Lock Detect is enumerated with cpuid Fn0000_0007_ECX_x0
bit [24 / BUSLOCKTRAP]. It can be enabled through MSR_IA32_DEBUGCTLMSR.
When enabled, hardware clears DR6[11] and raises a #DB exception on
occurrence of Bus Lock if CPL > 0. More detail about the feature can be
found in AMD APM[1].

Qemu supports remote debugging through host gdb (the "gdbstub" facility)
where some of the remote debugging features like instruction and data
breakpoints relies on the same hardware infrastructure (#DB, DR6 etc.)
that Bus Lock Detect also uses. Instead of handling internally, KVM
forwards #DB to Qemu when remote debugging is ON and #DB is being
intercepted. It's Qemu's responsibility to re-inject the exception to
guest when some of the exception source bits (in DR6) are not being
handled by Qemu remote debug handler. Bus Lock Detect is one such case.

[1]: AMD64 Architecture Programmer's Manual Pub. 40332, Rev. 4.07 - June
     2023, Vol 2, 13.1.3.6 Bus Lock Trap
     https://bugzilla.kernel.org/attachment.cgi?id=304653

Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
---
Kernel and KVM patches:
  https://lore.kernel.org/r/20240712093943.1288-1-ravi.bangoria@amd.com

 target/i386/cpu.h     | 1 +
 target/i386/kvm/kvm.c | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index c64ef0c1a2..89bcff2fa3 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -271,6 +271,7 @@ typedef enum X86Seg {
                 | CR4_SMEP_MASK | CR4_SMAP_MASK | CR4_PKE_MASK | CR4_PKS_MASK \
                 | CR4_LAM_SUP_MASK))
 
+#define DR6_BLD         (1 << 11)
 #define DR6_BD          (1 << 13)
 #define DR6_BS          (1 << 14)
 #define DR6_BT          (1 << 15)
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 6c864e4611..d128d4e5ca 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -5141,14 +5141,14 @@ static int kvm_handle_debug(X86CPU *cpu,
     } else if (kvm_find_sw_breakpoint(cs, arch_info->pc)) {
         ret = EXCP_DEBUG;
     }
-    if (ret == 0) {
+    if (ret == 0 || !(arch_info->dr6 & DR6_BLD)) {
         cpu_synchronize_state(cs);
         assert(env->exception_nr == -1);
 
         /* pass to guest */
         kvm_queue_exception(env, arch_info->exception,
                             arch_info->exception == EXCP01_DB,
-                            arch_info->dr6);
+                            ret == 0 ? arch_info->dr6 ^ DR6_BLD : DR6_BLD);
         env->has_error_code = 0;
     }
 
-- 
2.34.1