On 17.07.2025 18:23, 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>
Reviewed-by: Xiaoyao Li <xiaoyao...@intel.com>
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
This feels like a qemu-stable material, is it not?
Thanks,
/mjt
