linux-kernel - Re: [PATCH v7 4/8] KVM: x86/mmu: Migrate to __kvm_follow

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Date:   Thu, 24 Aug 2023 17:03:11 +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 4/8] KVM: x86/mmu: Migrate to __kvm_follow_pfn

On Thu, Jul 6, 2023 at 10:54 AM Isaku Yamahata <isaku.yamahata@...il.com> wrote:
>
> On Tue, Jul 04, 2023 at 04:50:49PM +0900,
> David Stevens <stevensd@...omium.org> wrote:
>
> > From: David Stevens <stevensd@...omium.org>
> >
> > Migrate from __gfn_to_pfn_memslot to __kvm_follow_pfn.
> >
> > Signed-off-by: David Stevens <stevensd@...omium.org>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 35 +++++++++++++++++++++++++----------
> >  1 file changed, 25 insertions(+), 10 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index ec169f5c7dce..e44ab512c3a1 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4296,7 +4296,12 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
> >  static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> >  {
> >       struct kvm_memory_slot *slot = fault->slot;
> > -     bool async;
> > +     struct kvm_follow_pfn foll = {
> > +             .slot = slot,
> > +             .gfn = fault->gfn,
> > +             .flags = FOLL_GET | (fault->write ? FOLL_WRITE : 0),
> > +             .allow_write_mapping = true,
> > +     };
> >
> >       /*
> >        * Retry the page fault if the gfn hit a memslot that is being deleted
> > @@ -4325,12 +4330,14 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> >                       return RET_PF_EMULATE;
> >       }
> >
> > -     async = false;
> > -     fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, false, &async,
> > -                                       fault->write, &fault->map_writable,
> > -                                       &fault->hva);
> > -     if (!async)
> > -             return RET_PF_CONTINUE; /* *pfn has correct page already */
> > +     foll.flags |= FOLL_NOWAIT;
> > +     fault->pfn = __kvm_follow_pfn(&foll);
> > +
> > +     if (!is_error_noslot_pfn(fault->pfn))
>
> We have pfn in struct kvm_follow_pfn as output. Can we make __kvm_follow_pfn()
> return int instead of kvm_pfn_t?  KVM_PFN_* seems widely used, though.

Switching __kvm_follow_pfn to return an int isn't difficult, but doing
is cleanly would require reworking the kvm_pfn_t/KVM_PFN_ERR_* api,
which as you said is quite widely used. That's a bit larger scope than
I want to do in this patch series.

-David