linux-kernel - Re: [Patch v3 0/7] Optimize clear dirty log

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Date:   Tue, 21 Mar 2023 13:40:21 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vipin Sharma <vipinsh@...gle.com>
Cc:     pbonzini@...hat.com, bgardon@...gle.com, dmatlack@...gle.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch v3 0/7] Optimize clear dirty log

On Tue, Mar 21, 2023, Sean Christopherson wrote:
> On Tue, Mar 21, 2023, Vipin Sharma wrote:
> > On Mon, Mar 20, 2023 at 5:41 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > On Fri, Mar 17, 2023, Sean Christopherson wrote:
> > > > Did a cursory glance, looks good.  I'll do a more thorough pass next week and get
> > > > it queued up if all goes well.  No need for a v4 at this point, I'll fixup David's
> > > > various nits when applying.
> > >
> > > Ooof, that ended up being painful.  In hindsight, I should have asked for a v4,
> > > but damage done, and it's my fault for throwing you a big blob of code in the
> > > first place.
> > >
> > > I ended up splitting the "interesting" patches into three each:
> > >
> > >   1. Switch to the atomic-AND
> > >   2. Drop the access-tracking / dirty-logging (as appropriate)
> > >   3. Drop the call to __handle_changed_spte()
> > >
> > > because logically they are three different things (although obviously related).
> > >
> > > I have pushed the result to kvm-x86/mmu, but haven't merged to kvm-x86/next or
> > > sent thanks because it's not yet tested.  I'll do testing tomorrow, but if you
> > > can take a look in the meantime to make sure I didn't do something completely
> > > boneheaded, it'd be much appreciated.
> > 
> > 
> > Thanks for refactoring the patches. I reviewed the commits, no obvious
> > red flags from my side. Few small nits I found:
> > 
> > commit e534a94eac07 ("KVM: x86/mmu: Use kvm_ad_enabled() to determine
> > if TDP MMU SPTEs need wrprot")
> >  - kvm_ad_enabled() should be outside the loop.
> 
> Hmm, I deliberately left it inside the loop, but I agree that it would be better
> to hoist it out in that commit.
> 
> > commit 69032b5d71ef (" KVM: x86/mmu: Atomically clear SPTE dirty state
> > in the clear-dirty-log flow")
> >  - MMU_WARN_ON(kvm_ad_enabled() &&
> > spte_ad_need_write_protect(iter.old_spte) should be after
> > if(iter.level > PG_LEVEL_4k...)
> 
> Ah, hrm.  This was also deliberate, but looking at the diff I agree that relative
> to the diff, it's an unnecessary/unrelated change.  I think what I'll do is
> land the assertion above the "if (iter.level > PG_LEVEL_4K ||" in the above
> commit that switches to kvm_ad_enabled().  That way there shouldn't be any change
> for the assertion in this commit.

Aha!  Even better, split this into yet one more patch to dedup the guts before
switching to the atomic-AND, and give clear_dirty_gfn_range() the same treatment.
That further isolates the changes, provides solid justification for hoisting the
kvm_ad_enabled() check out of the loop (it's basically guaranteed to be a single
memory read that hits the L1), and keeps clear_dirty_gfn_range() and
clear_dirty_pt_masked() as similar as is reasonably possible.

Speaking of which, I'll send a patch to remove the redundant is_shadow_present_pte()
check in clear_dirty_gfn_range(), that's already handled by tdp_root_for_each_leaf_pte().