linux-kernel - Re: [Patch] KVM: x86/mmu: Make optimized __handle_changed

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Message-ID: <CAHVum0eReWgo_3yWxdtbyFGxeTnWUiEn9uVu0W5XX3KfHAgikw@mail.gmail.com>
Date:   Mon, 30 Jan 2023 12:17:19 -0800
From:   Vipin Sharma <vipinsh@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Ben Gardon <bgardon@...gle.com>,
        David Matlack <dmatlack@...gle.com>, pbonzini@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch] KVM: x86/mmu: Make optimized __handle_changed_spte() for
 clear dirty log

On Mon, Jan 30, 2023 at 10:09 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Sat, Jan 28, 2023, Vipin Sharma wrote:
> > On Wed, Jan 25, 2023 at 5:49 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > -static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> > > -                               u64 old_spte, u64 new_spte, int level,
> > > -                               bool shared)
> > > -{
> > > -       __handle_changed_spte(kvm, as_id, gfn, old_spte, new_spte, level,
> > > -                             shared);
> > >         handle_changed_spte_acc_track(old_spte, new_spte, level);
> > > -       handle_changed_spte_dirty_log(kvm, as_id, gfn, old_spte,
> > > -                                     new_spte, level);
> > > +
> > > +       /* COMMENT GOES HERE. */
> >
> > Current "shared" callers are not making a page dirty. If a new
> > "shared" caller makes a page dirty then make sure
> > handle_changed_spte_dirty_log is called.
> >
> > How is this?
>
> I was hoping for a more definitive "rule" than "KVM doesn't currently do XYZ".
>
> > > +       if (!shared)
> > > +               handle_changed_spte_dirty_log(kvm, as_id, gfn, old_spte,
> > > +                                             new_spte, level);
> > >  }
> > >

What if implementation is changed a little more? I can think of two options:

Option 1:
Remove handle_changed_spte_dirty_log() and let the callers handle
mark_page_dirty_in_slot(). Similar to how fast page faults do this.
This will get rid of the "shared" variable and defining its rules for
the shared and nonshared flow.

Option 2:
Changing meaning of this variable from "shared" to something like
"handle_dirty_log"
Callers will know if they want dirty log to be handled or not.

I am preferring option 1.