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

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Message-ID: <Y9hJg9hDQFD3X720@google.com>
Date:   Mon, 30 Jan 2023 22:49:39 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vipin Sharma <vipinsh@...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, Vipin Sharma wrote:
> 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.

Sorry, what I meant by "definitive rule" was an explanation of why KVM doesn't
need to do dirty log tracking for tdp_mmu_set_spte_atomic().

Figured things out after a bit o' archaeology.  Commit bcc4f2bc5026 ("KVM: MMU:
mark page dirty in make_spte") shifted the dirtying of the memslot to make_spte(),
and then commit 6ccf44388206 ("KVM: MMU: unify tdp_mmu_map_set_spte_atomic and
tdp_mmu_set_spte_atomic_no_dirty_log") covered up the crime.

Egad!  I believe that means handle_changed_spte_dirty_log() is dead code for all
intents and purposes, as there is no path that creates a WRITABLE 4KiB SPTE without
bouncing through make_spte().  set_spte_gfn() => kvm_mmu_changed_pte_notifier_make_spte()
only creates !WRITABLE SPTEs, ignoring for the moment that that's currently dead
code too (see commit c13fda237f08 ("KVM: Assert that notifier count is elevated in
.change_pte()")).

So I _think_ we can do option #1 simply by deleting code.