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

[Sean Christopherson <seanjc@google.com>]

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).

> But I am not sure if it's worth it. I'll defer to maintainers.