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

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Jun 27, 2025 at 08:23:52AM +0200, Alexandre Chartre wrote:
> 
> On 6/27/25 07:41, Xiaoyao Li wrote:
> > On 6/26/2025 10:02 PM, Sean Christopherson wrote:
> > > +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.
> > 
> > Isn't it the Windows bugs? I think it is incorrect to assume AMD will never implement ARCH_CAPABILITIES.
> > 
> 
> Yes, although on one hand they are just following the current AMD specification which
> says that ARCH_CAPABILITIES is not defined on AMD cpus; but on the other hand they are
> breaking a 6+ years behavior. So it might be nice if we could prevent such an issue in
> the future.

Hi Sean,

Part of the virtualization stack is to lie accurately and in this case
KVM is doing it incorrectly. Not fixing it b/c of it being for 7 years
in and being part of an ABI but saying it should be fixed in QEMU sounds
like you agree technically, but are constrained by a policy.

N.B.  Also the TSC deadline MSR is advertised yet AMD does not support
it.

Looping in Linus here. Linus, thoughts?
> 
> Note that a Windows update preview has just been released with a fix (OS Build 26100.4484),
> but the Windows automatic update will still install the version with the issue at the moment
> (automatic update doesn't install preview).
> 
> alex.
>