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

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Message-ID: <CAD=HUj5ermRAjxVYhERDA7fE0cZ5TAGunP6j7zM5YC6PyiZh-g@mail.gmail.com>
Date:   Thu, 6 Jul 2023 14:18:06 +0900
From:   David Stevens <stevensd@...omium.org>
To:     Isaku Yamahata <isaku.yamahata@...il.com>
Cc:     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 Thu, Jul 6, 2023 at 11:10 AM Isaku Yamahata <isaku.yamahata@...il.com> wrote:
>
> On Tue, Jul 04, 2023 at 04:50:50PM +0900,
> David Stevens <stevensd@...omium.org> wrote:
>
> > diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> > index cf2c6426a6fc..46c681dc45e6 100644
> > --- a/arch/x86/kvm/mmu/spte.c
> > +++ b/arch/x86/kvm/mmu/spte.c
> > @@ -138,7 +138,7 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> >              const struct kvm_memory_slot *slot,
> >              unsigned int pte_access, gfn_t gfn, kvm_pfn_t pfn,
> >              u64 old_spte, bool prefetch, bool can_unsync,
> > -            bool host_writable, u64 *new_spte)
> > +            bool host_writable, bool is_refcounted, u64 *new_spte)
> >  {
> >       int level = sp->role.level;
> >       u64 spte = SPTE_MMU_PRESENT_MASK;
> > @@ -188,6 +188,8 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> >
> >       if (level > PG_LEVEL_4K)
> >               spte |= PT_PAGE_SIZE_MASK;
> > +     else if (is_refcounted)
> > +             spte |= SPTE_MMU_PAGE_REFCOUNTED;
>
> Is REFCOUNTED for 4K page only?  What guarantees that large page doesn't have
> FOLL_GET? or can we set the bit for large page?

Oh, you're right, it should apply to >4K pages as well. This was based
on stale thinking from earlier versions of this series.

-David