Re: [RFC PATCH 3/4] KVM: stats: Add ioctl commands to pull statistics in binary format

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On Wed, Mar 10, 2021 at 11:44 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 10/03/21 18:31, Marc Zyngier wrote:
> >> Maintaining VM-global counters would require an atomic instruction and
> >> would suffer lots of cacheline bouncing even on architectures that
> >> have relaxed atomic memory operations.
> > Which is why we have per-cpu counters already. Making use of them
> > doesn't seem that outlandish.
>
> But you wouldn't be able to guarantee consistency anyway, would you?
> You *could* copy N*M counters to userspace, but there's no guarantee
> that they are consistent, neither within a single vCPU nor within a
> single counter.
>
> >> Speed/efficiency of retrieving statistics is important, but let's keep
> >> in mind that the baseline for comparison is hundreds of syscalls and
> >> filesystem lookups.
> >
> > Having that baseline in the cover letter would be a good start, as
> > well as an indication of the frequency this is used at.
>
> Can't disagree, especially on the latter which I have no idea about.
>
> Paolo
>
Marc, Paolo, thanks for the comments. I will add some more information
in the cover letter.

Thanks,
Jing



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