Re: kvm selftest 'msr' fails on some skylake cpus

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 13, 2024, Maxim Levitsky wrote:
> Hi!
> 
> This kvm unit test tests that all reserved bits of the MSR_IA32_FLUSH_CMD #GP, but apparently
> on some systems this test fails.
> 
> For example I reproduced this on:
> 
> model name	: Intel(R) Xeon(R) CPU E3-1260L v5 @ 2.90GHz
> stepping	: 3
> microcode	: 0xf0
> 
> 
> As I see in the 'vmx_vcpu_after_set_cpuid', we passthough this msr to the guest AS IS,
> thus the unit test tests the microcode.
> 
> So I suspect that the test actually caught a harmless microcode bug.

Yeah, we encountered the same thing and came to the same conclusion.

> What do you think we should do to workaround this? Maybe disable this check on
> affected cpus or turn it into a warning because MSR_IA32_FLUSH_CMD reserved bits
> test doesn't test KVM?

Ya, Mingwei posted a patch[*] to force KVM to emulate the faulting accesses, which
more or less does exactly that, but preserves a bit of KVM coverage.  I'll get a
KUT pull request sent to Paolo today, I've got a sizeable number of changes ready.

[*] https://lore.kernel.org/all/20240417232906.3057638-3-mizhang@google.com