Re: [PATCH] KVM: x86/cpuid: Clear XFD for component i if the base feature is missing

[Paolo Bonzini]

On 1/18/22 07:43, Like Xu wrote:
> On 18/1/2022 1:31 am, Paolo Bonzini wrote:
> > On 1/17/22 08:45, Like Xu wrote:
> > > From: Like Xu <likexu@xxxxxxxxxxx>
> > >
> > > According to Intel extended feature disable (XFD) spec, the 
> > > sub-function i
> > > (i > 1) of CPUID function 0DH enumerates "details for state component i.
> > > ECX[2] enumerates support for XFD support for this state component."
> > >
> > > If KVM does not report F(XFD) feature (e.g. due to CONFIG_X86_64),
> > > then the corresponding XFD support for any state component i
> > > should also be removed. Translate this dependency into KVM terms.
> > >
> > > Fixes: 690a757d610e ("kvm: x86: Add CPUID support for Intel AMX")
> > > Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> > > ---
> > >   arch/x86/kvm/cpuid.c | 3 +++
> > >   1 file changed, 3 insertions(+)
> > >
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index c55e57b30e81..e96efef4f048 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -886,6 +886,9 @@ static inline int __do_cpuid_func(struct 
> > > kvm_cpuid_array *array, u32 function)
> > >                   --array->nent;
> > >                   continue;
> > >               }
> > > +
> > > +            if (!kvm_cpu_cap_has(X86_FEATURE_XFD))
> > > +                entry->ecx &= ~BIT_ULL(2);
> > >               entry->edx = 0;
> > >           }
> > >           break;
> >
> > Generally this is something that is left to userspace.  Apart from the 
> > usecase of "call KVM_GET_SUPPORTED_CPUID and pass it to 
> > KVM_SET_CPUID2", userspace should know what any changed bits mean.
> >
> > Paolo
>
> I totally agree that setting the appropriate CPUID bits for a feature is 
> a user space thing.
>
> But this patch is more focused on fixing a different type of problem, 
> which is
> that the capabilities exposed by KVM should not *contradict each other* :
>
>      a user space may be confused with the current code base :
>      - why KVM does not have F(XFD) feature (MSR_IA32_XFD and XFD_ERR 
> non-exit),
>      - but KVM reports XFD support for state component i individually;

Got it.  Yeah, the patch makes sense for the sake of CONFIG_X86_64.

Paolo

> This is like KVM reporting PEBS when no PMU capacity is reported (due to 
> module param).