Re: [PULL 1/6] target/i386: do not expose ARCH_CAPABILITIES on AMD CPU

[Paolo Bonzini <pbonzini@redhat.com>]

On 7/18/25 09:21, Michael Tokarev wrote:
> 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.chartre@oracle.com>
>> Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
>> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> 
> This feels like a qemu-stable material, is it not?
Yes, good idea.

Paolo