linux-kernel - Re: [PATCH 2/2] selftests: KVM: Add page splitting test

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Date:   Tue, 24 Jan 2023 09:15:39 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Ricardo Koller <ricarkol@...gle.com>
Cc:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vipin Sharma <vipinsh@...gle.com>
Subject: Re: [PATCH 2/2] selftests: KVM: Add page splitting test

On Tue, Jan 24, 2023 at 7:54 AM Ricardo Koller <ricarkol@...gle.com> wrote:
>
> On Mon, Jan 23, 2023 at 10:41:18AM -0800, David Matlack wrote:
> >
> > Ricardo, if you're interested in adding page size stats to KVM/ARM ahead
> > of the Common MMU, e.g. to test eager page splitting, let me know.
>
> Sure, I can do that. Sounds pretty useful too.
>
> > I
> > want to make sure we align on the userspace-visible stat names to avoid
> > churn down the road. Specifically, do we want to expose neutral names
> > like pages_{pte,pmd,pud} or expand the KVM/x86 list to include all of
> > ARM's possible pages sizes like pages_{4k,16k,64k,...} (or both)?
>
> I would prefer the latter, mainly to match the x86 names:
>
>         +       stats->pages_4k = vm_get_stat(vm, "pages_4k");
>         +       stats->pages_2m = vm_get_stat(vm, "pages_2m");
>         +       stats->pages_1g = vm_get_stat(vm, "pages_1g");
>         (from this patch)

We can always add pages_{pte,pmd,pud} to x86 as aliases of the
existing stats. The series I recently sent out to allow custom names
for stats [1] would make adding the aliases trivial actually.

[1] https://lore.kernel.org/kvm/20230118175300.790835-1-dmatlack@google.com/

>
> but pages_{pte,pmd,pud} would certainly make this test simpler
> as it would handle all guest page sizes:
>
>         +       stats->pages_pte = vm_get_stat(vm, "pages_pte");
>

Yeah pages_{pte,pmd,pud} would certainly make the test simpler.

At this point I'm leaning toward pages_{pte,pmd,pud} to unblock this
testing in an architecture neutral way, and we can add
pages_{4k,16k,...} to ARM in the future using aliases.