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

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Date:   Mon, 23 Jan 2023 10:41:18 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     Ricardo Koller <ricarkol@...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 Fri, Jan 20, 2023 at 12:04:04PM -0800, Ben Gardon wrote:
> On Fri, Jan 20, 2023 at 6:34 AM Ricardo Koller <ricarkol@...gle.com> wrote:
> >
> ...
> > > > > > +
> > > > > > +     run_test(&p);
> > > > >
> > > > > Use for_each_guest_mode() to run against all supported guest modes.
> > > >
> > > > I'm not sure that would actually improve coverage. None of the page
> > > > splitting behavior depends on the mode AFAICT.
> > >
> > > You need to use for_each_guest_mode() for the ARM case. The issue is
> > > that whatever mode (guest page size and VA size) you pick might not be
> > > supported by the host. So, you first to explore what's available (via
> > > for_each_guest_mode()).
> >
> > Actually, that's fixed by using the default mode, which picks the
> > first available
> > mode. I would prefer to use for_each_guest_mode() though, who knows and
> > something fails with some specific guest page size for some reason.
> 
> Okay, will do. I wasn't sure if we did eager page splitting on ARM, so

Ricardo is in the process of upstreaming eager page splitting for ARM:

https://lore.kernel.org/kvm/20230113035000.480021-1-ricarkol@google.com/

> I was only planning on making this test for x86_64 initially, hence it
> being in that directory. If ARM rolls with the same behavior, then
> I'll add the for_each_mode bit and move the test up a directory.

In addition to for_each_guest_mode(), KVM/ARM will need to expose page
size stats so the test can verify the splitting (yet another reason to
have a common MMU).

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. 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)?