[Bug 218739] pmu_counters_test kvm-selftest fails with (count != NUM_INSNS_RETIRED)

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=218739

--- Comment #3 from Sean Christopherson (seanjc@google.com) ---
On Mon, May 27, 2024, bugzilla-daemon@kernel.org wrote:
> I also see this test fail sometimes (once per hour or so of continuous
> running)
>  and in my case it fails because 'count != 0' assert on
> INTEL_ARCH_LLC_MISSES_INDEX event and only for this event.
> 
> The reason is IMHO, is that it is possible  to have 0 LLC misses if the cache
> is large enough and code was run for enough iterations.

The test does CLFUSH{,OPT} on its future code sequence after enabling the
counter.
In theory, that's should guarantee an LLC Miss.

Hmm, but this SDM blurb about speculative loads makes me think past me was
wrong.

  (that is, data can be speculatively loaded into a cache line just before,
during,
   or after the execution of a CLFLUSH instruction that references the cache
line).

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