Re: [PATCH] kvm/x86/vmx: Make the emulation of MSR_IA32_ARCH_CAPABILITIES only for vmx

[Paolo Bonzini <pbonzini@redhat.com>]

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