Re: [PATCH v3 5/5] KVM: selftests: KVM: SVM: Add Idle HLT intercept test

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 13, 2024, Sean Christopherson wrote:
> On Fri, May 31, 2024, Chao Gao wrote:
> > On Thu, May 30, 2024 at 06:49:56PM +0530, Manali Shukla wrote:
> > >>> +	/* Check the extension for binary stats */
> > >>> +	TEST_REQUIRE(this_cpu_has(X86_FEATURE_IDLE_HLT));
> > >> 
> > >> IIUC, this test assumes that the IDLE_HLT feature is enabled for guests if it
> > >> is supported by the CPU. But this isn't true in some cases:
> > >> 
> > >I understand you are intending to create a capability for IDLE HLT intercept
> > >feature, but in my opinion, the IDLE Halt intercept feature doesn't require
> > >user space to do anything for the feature itself.
> > 
> > Yes, I agree. Actually, I was thinking about:
> > 
> > 1. make the feature bit visible from /proc/cpuinfo by removing the leading ""
> >    from the comment following the bit definition in patch 1
> > 
> > 2. parse /proc/cpuinfo to determine if this IDLE_HLT feature is supported by the
> >    kernel
> 
> Neither of these is sufficient/correct.  E.g. they'll get false positives if run
> on a kernel that recognizes IDLE_HLT, but that doesn't have KVM support for
> enabling the feature.
> 
> The canonical way to check for features in KVM selftests is kvm_cpu_has(), which
> looks at KVM_GET_SUPPORTED_CPUID (by default, selftests VMs enable all features,
> i.e. reflect the result of KVM_GET_SUPPORTED_CPUID into KVM_SET_CPUID2).

Never mind, brain fart.  That only works for features KVM is exposing to the guest,
this is purely a KVM/host feature.

That said, doesn't this test also require that AVIC be enabled?  Oh, or does this
happen to work because KVM uses V_INTR to detect interrupt windows?