linux-kernel - Re: [PATCH v7 5/8] KVM: x86/mmu: Don't pass FOLL_GET to __kvm_follow

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Date:   Tue, 11 Jul 2023 11:59:08 +0900
From:   David Stevens <stevensd@...omium.org>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Xu <peterx@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v7 5/8] KVM: x86/mmu: Don't pass FOLL_GET to __kvm_follow_pfn

On Tue, Jul 11, 2023 at 1:34 AM Isaku Yamahata <isaku.yamahata@...il.com> wrote:
>
> On Fri, Jul 07, 2023 at 10:35:02AM +0900,
> David Stevens <stevensd@...omium.org> wrote:
>
> > > > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > > > > index e44ab512c3a1..b1607e314497 100644
> > > > > > --- a/arch/x86/kvm/mmu/mmu.c
> > > > > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > > >
> > > > > ...
> > > > >
> > > > > > @@ -2937,6 +2943,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
> > > > > >       bool host_writable = !fault || fault->map_writable;
> > > > > >       bool prefetch = !fault || fault->prefetch;
> > > > > >       bool write_fault = fault && fault->write;
> > > > > > +     bool is_refcounted = !fault || fault->is_refcounted_page;
> > > > >
> > > > > Just wonder, what if a non-refcounted page is prefetched?  Or is it possible in
> > > > > practice?
> > > >
> > > > Prefetching is still done via gfn_to_page_many_atomic, which sets
> > > > FOLL_GET. That's fixable, but it's not something this series currently
> > > > does.
> > >
> > > So if we prefetch a page, REFCOUNTED bit is cleared unconditionally with this
> > > hunk.  kvm_set_page_{dirty, accessed} won't be called as expected for prefetched
> > > spte.  If I read the patch correctly, REFCOUNTED bit in SPTE should represent
> > > whether the corresponding page is ref-countable or not, right?
> > >
> > > Because direct_pte_prefetch_many() is for legacy KVM MMU and FNAME(prefetch_pte)
> > > is shadow paging, we need to test it with legacy KVM MMU or shadow paging to hit
> > > the issue, though.
> > >
> >
> > direct_pte_prefetch_many and prefetch_gpte both pass NULL for the
> > fault parameter, so is_refcounted will evaluate to true. So the spte's
> > refcounted bit will get set in that case.
>
> Oops, my bad.  My point is "unconditionally".  Is the bit always set for
> non-refcountable pages?  Or non-refcountable pages are not prefeched?

The bit is never set for non-refcounted pages, and is always set for
refcounted pages. The current series never prefetches non-refcounted
pages, since it continues to use the gfn_to_page_many_atomic API.

-David