Re: [PATCH v4 15/17] kvm: x86: Report CORE_CAPABILITY on GET_SUPPORTED_CPUID

Xiaoyao Li
2019-03-05T07:03:28Z

On Mon, 2019-03-04 at 12:14 +0100, Paolo Bonzini wrote:
> On 04/03/19 12:10, Xiaoyao Li wrote:
> > Like you said before, I think we don't need the condition judgment
> > "if(boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))", but to set
> > F(CORE_CAPABILITY)
> > always for guest since MSR_IA32_CORE_CAPABILITY is emulated.
> >
> > And we should set the right emulated value of MSR_IA32_CORE_CAPABILITY for
> > guest
> > in function kvm_get_core_capability() based on whether
> > boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) just as you commented in the
> > next
> > patch.
>
> Yes, that would certainly be better. However, you'd also have to move
> MSR_IA32_CORE_CAPABILITY handling to x86.c, because you'd have to enable
> X86_FEATURE_CORE_CAPABILITY for AMD.
>
> Paolo

Hi, Paolo

I just notice that F(ARCH_CAPABILITIES) is set unconditionally. However the
handling of MSR_IA32_ARCH_CAPABILITIES only exists with vmx, and the emulation
of this MSR is in vmx->arch_capabilities.
These will cause #GP when guest kernel rdmsr(MSR_IA32_ARCH_CAPABILITES) with AMD
CPU since there is handling for svm.
Maybe what I think is not correct due to my limit knowledge of
MSR_IA32_ARCH_CAPABILITIES and how kernel handles its related features.

If what I said above is true and it's indeed an issue. So based on the fact that
both MSR_IA32_ARCH_CAPABILITIES and MSR_IA32_CORE_CAPABILITY are feature-
enumerating MSR and we emulate them in KVM, there are 2 choices for us to handle
it:
1. we unconditionally set F(ARCH_CAPABILITIES) and F(CORE_CAPABILITY) for guest,
move the emulation of these 2 MSRs to vcpu->arch.***, and move all the handling
of these 2 MSRs to x86.c.

2. we conditionally set F(ARCH_CAPABILITIES) and F(CORE_CAPABILITY) only if it
is intel CPU. So we just need to emulate these 2 MSRs in vmx->*** for intel CPU.

I prefer option 2 personally for CORE_CAPABILITY since it makes no sense to
expose MSR_IA32_CORE_CAPABILITY to other x86 vendors.
About ARCH_CAPABILITIES, it seems that we emulate it for generic x86 cpus that
!x86_match_cpu(cpu_no_speculation). So we should choose option 1, to move the
emulation and handling to x86.c?

Xiaoyao