linux-kernel - Re: [PATCH] KVM: selftests: replace assertion with warning in access_tracking_perf

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Message-ID: <005c8afa-d290-d140-0dac-19a41f2ef81a@redhat.com>
Date:   Tue, 27 Sep 2022 14:14:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Shuah Khan <shuah@...nel.org>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: selftests: replace assertion with warning in
 access_tracking_perf_test

On 9/26/22 10:29, Emanuele Giuseppe Esposito wrote:
> Page_idle uses {ptep/pmdp}_clear_young_notify which in turn calls
> the mmu notifier callback ->clear_young(), which purposefully
> does not flush the TLB.
> 
> When running the test in a nested guest, point 1. of the test
> doc header is violated, because KVM TLB is unbounded by size
> and since no flush is forced, KVM does not update the sptes
> accessed/idle bits resulting in guest assertion failure.
> 
> More precisely, only the first ACCESS_WRITE in run_test() actually
> makes visible changes, because sptes are created and the accessed
> bit is set to 1 (or idle bit is 0). Then the first mark_memory_idle()
> passes since access bit is still one, and sets all pages as idle
> (or not accessed). When the next write is performed, the update
> is not flushed therefore idle is still 1 and next mark_memory_idle()
> fails.

Queued, thanks.

Paolo