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

[Like Xu]

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;

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

Does this clarification help ?

Thanks,
Like Xu