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

[Alexandre Chartre <alexandre.chartre@oracle.com>]

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.

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.