linux-kernel - Re: [PATCH 1/1] selftests: KVM: add test to print boottime wallclock

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Message-ID: <ZTA3W-f4sOX3LHfi@google.com>
Date:   Wed, 18 Oct 2023 12:51:55 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Dongli Zhang <dongli.zhang@...cle.com>
Cc:     kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        shuah@...nel.org, dwmw2@...radead.org, joe.jin@...cle.com
Subject: Re: [PATCH 1/1] selftests: KVM: add test to print boottime wallclock

On Fri, Oct 06, 2023, Dongli Zhang wrote:
> As inspired by the discussion in [1], the boottime wallclock may drift due
> to the fact that the masterclock (or host monotonic clock) and kvmclock are
> calculated based on the algorithms in different domains.
> 
> This is to introduce a testcase to print the boottime wallclock
> periodically to help diagnose the wallclock drift issue in the future.
> 
> The idea is to wrmsr the MSR_KVM_WALL_CLOCK_NEW, and read the boottime
> wallclock nanoseconds immediately.

This doesn't actually test anything of interest though.  IIUC, it requires a human
looking at the output for it to provide any value.  And it requires a manual
cancelation, which makes it even less suitable for selftests.

I like the idea, e.g. I bet there are more utilities that could be written that
utilize the selftests infrastructure, just not sure what to do with this (assuming
it can't be massaged into an actual test).