Re: [PATCH] i386/cpu: ARCH_CAPABILITIES should not be advertised on AMD

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Jul 01, 2025 at 03:59:12PM -0400, Konrad Rzeszutek Wilk wrote:
> ..snip..
> > OK, back to the original question "what should the code do?"
> > 
> > My answer is, it can behave with any of below option:
> > 
> > - Be vendor agnostic and stick to x86 architecture. If CPUID enumerates a
> > feature, then the feature is available architecturally.
> 
> Exactly. That is what we believe Windows does.
> 
> 
> By this logic KVM is at fault for exposing this irregardless of the
> platform (when using -cpu host). And Sean (the KVM maintainer) agrees it is
> a bug. But he does not want it in the kernel due to guest ABI and hence
> the ask is to put this in QEMU.

If QEMU unconditionally disables this on AMD, and a future AMD CPU
does implement it, then QEMU is now broken because it won't be fully
exposing valid features impl by the host CPU and supported by KVM.

IOW, if we're going to have QEMU workaround the KVM mistake, then
the code change needs to be more refined.

QEMU needs to first check whether the host CPU implements
ARCH_CAPABILITIES and conditionally disable it in the guest CPU
based on that host CPU check. Of course that would re-expose the
Windows guest bug, but that ceases to be KVM/QEMU's problem at
that point, as we'd be following a genuine physical CPU impl.

With regards,
Daniel
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