Re: [PATCH] target/i386: do not expose ARCH_CAPABILITIES on AMD CPU

[Zhao Liu <zhao1.liu@intel.com>]

On Thu, Jul 17, 2025 at 12:34:48PM +0200, Paolo Bonzini wrote:
> Date: Thu, 17 Jul 2025 12:34:48 +0200
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: [PATCH] target/i386: do not expose ARCH_CAPABILITIES on AMD CPU
> X-Mailer: git-send-email 2.50.1
> 
> 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.

Make sense.

> Reported-by: Alexandre Chartre <alexandre.chartre@oracle.com>
> Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  target/i386/kvm/kvm.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)

Good to see this fix. Late but,

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>