Re: [PATCH] kvm/x86: ARCH_CAPABILITIES should not be advertised on AMD

[Sean Christopherson <seanjc@google.com>]

+Jim

For the scope, "KVM: x86:"

On Thu, Jun 26, 2025, Alexandre Chartre wrote:
> KVM emulates the ARCH_CAPABILITIES on x86 for both vmx and svm.
> However the IA32_ARCH_CAPABILITIES MSR is an Intel-specific MSR
> so it makes no sense to emulate it on AMD.
> 
> The AMD documentation specifies that this MSR is not defined on
> the AMD architecture. So emulating this MSR on AMD can even cause
> issues (like Windows BSOD) as the guest OS might not expect this
> MSR to exist on such architecture.
> 
> Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
> ---
> 
> A similar patch was submitted some years ago but it looks like it felt
> through the cracks:
> https://lore.kernel.org/kvm/20190307093143.77182-1-xiaoyao.li@linux.intel.com/

It didn't fall through the cracks, we deliberately elected to emulate the MSR in
common code so that KVM's advertised CPUID support would match KVM's emulation.

  On Thu, 2019-03-07 at 19:15 +0100, Paolo Bonzini wrote:
  > On 07/03/19 18:37, Sean Christopherson wrote:
  > > On Thu, Mar 07, 2019 at 05:31:43PM +0800, Xiaoyao Li wrote:
  > > > At present, we report F(ARCH_CAPABILITIES) for x86 arch(both vmx and svm)
  > > > unconditionally, but we only emulate this MSR in vmx. It will cause #GP
  > > > while guest kernel rdmsr(MSR_IA32_ARCH_CAPABILITIES) in an AMD host.
  > > > 
  > > > Since MSR IA32_ARCH_CAPABILITIES is an intel-specific MSR, it makes no
  > > > sense to emulate it in svm. Thus this patch chooses to only emulate it
  > > > for vmx, and moves the related handling to vmx related files.
  > > 
  > > What about emulating the MSR on an AMD host for testing purpsoes?  It
  > > might be a useful way for someone without Intel hardware to test spectre
  > > related flows.
  > > 
  > > In other words, an alternative to restricting emulation of the MSR to
  > > Intel CPUS would be to move MSR_IA32_ARCH_CAPABILITIES handling into
  > > kvm_{get,set}_msr_common().  Guest access to MSR_IA32_ARCH_CAPABILITIES
  > > is gated by X86_FEATURE_ARCH_CAPABILITIES in the guest's CPUID, e.g.
  > > RDMSR will naturally #GP fault if userspace passes through the host's
  > > CPUID on a non-Intel system.
  > 
  > This is also better because it wouldn't change the guest ABI for AMD
  > processors.  Dropping CPUID flags is generally not a good idea.
  > 
  > Paolo

I don't necessarily disagree about emulating ARCH_CAPABILITIES being pointless,
but Paolo's point about not changing ABI for existing setups still stands.  This
has been KVM's behavior for 6 years (since commit 0cf9135b773b ("KVM: x86: Emulate
MSR_IA32_ARCH_CAPABILITIES on AMD hosts"); 7 years, if we go back to when KVM
enumerated support without emulating the MSR (commit 1eaafe91a0df ("kvm: x86:
IA32_ARCH_CAPABILITIES is always supported").

And it's not like KVM is forcing userspace to enumerate support for
ARCH_CAPABILITIES, e.g. QEMU's named AMD configs don't enumerate support.  So
while I completely agree KVM's behavior is odd and annoying for userspace to deal
with, this is probably something that should be addressed in userspace.

> I am resurecting this change because some recent Windows updates (like OS Build
> 26100.4351) crashes on AMD KVM guests (BSOD with Stop code: UNSUPPORTED PROCESSOR)
> just because the ARCH_CAPABILITIES is available.
> 
> ---
>  arch/x86/kvm/svm/svm.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index ab9b947dbf4f..600d2029156e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -5469,6 +5469,9 @@ static __init void svm_set_cpu_caps(void)
>  
>  	/* Don't advertise Bus Lock Detect to guest if SVM support is absent */
>  	kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT);
> +
> +	/* Don't advertise ARCH_CAPABILITIES on AMD */
> +	kvm_cpu_cap_clear(X86_FEATURE_ARCH_CAPABILITIES);

Strictly speaking, I think we'd want to update svm_has_emulated_msr() as well.

>  }
>  
>  static __init int svm_hardware_setup(void)
> -- 
> 2.43.5
>