On 7/17/2025 6:34 PM, Paolo Bonzini wrote:
KVM emulates the ARCH_CAPABILITIES on x86 for both Intel and AMD
cpus, although the IA32_ARCH_CAPABILITIES MSR is an Intel-specific
MSR and it makes no sense to emulate it on AMD.
As a consequence, VMs created on AMD with qemu -cpu host and using
KVM will advertise the ARCH_CAPABILITIES feature and provide the
IA32_ARCH_CAPABILITIES MSR. This can cause issues (like Windows BSOD)
as the guest OS might not expect this MSR to exist on such cpus (the
AMD documentation specifies that ARCH_CAPABILITIES feature and MSR
are not defined on the AMD architecture).
A fix was proposed in KVM code, however KVM maintainers don't want to
change this behavior that exists for 6+ years and suggest changes to be
done in QEMU instead. Therefore, hide the bit from "-cpu host":
migration of -cpu host guests is only possible between identical host
kernel and QEMU versions, therefore this is not a problematic breakage.
If a future AMD machine does include the MSR, that would re-expose the
Windows guest bug; but it would not be KVM/QEMU's problem at that
point, as we'd be following a genuine physical CPU impl.
Reported-by: Alexandre Chartre <alexandre.char...@oracle.com>
Suggested-by: Daniel P. Berrangé <berra...@redhat.com>
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
Reviewed-by: Xiaoyao Li <xiaoyao...@intel.com>
This looks reasonable to me.
Though it will break the usecase of people boot VM with "-cpu
AMD-model,+arch_capabilities", like in [1], I think it's OK break it and
force people to not do such thing any more.
[1] https://gitlab.com/qemu-project/qemu/-/issues/3001
---
target/i386/kvm/kvm.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index e8c8be09bae..369626f8c8d 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -503,8 +503,12 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s,
uint32_t function,
* Linux v4.17-v4.20 incorrectly return ARCH_CAPABILITIES on SVM
hosts.
* We can detect the bug by checking if MSR_IA32_ARCH_CAPABILITIES is
* returned by KVM_GET_MSR_INDEX_LIST.
+ *
+ * But also, because Windows does not like ARCH_CAPABILITIES on AMD
+ * mcahines at all, do not show the fake ARCH_CAPABILITIES MSR that
+ * KVM sets up.
*/
- if (!has_msr_arch_capabs) {
+ if (!has_msr_arch_capabs || !(edx & CPUID_7_0_EDX_ARCH_CAPABILITIES)) {
ret &= ~CPUID_7_0_EDX_ARCH_CAPABILITIES;
}
} else if (function == 7 && index == 1 && reg == R_EAX) {
