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

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Date:   Tue, 21 Mar 2023 12:57:20 -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, 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.

> commit 93c375bb6aea ("KVM: x86/mmu: Bypass __handle_changed_spte()
> when clearing TDP MMU dirty bits")
>  - Needs new performance numbers. Adding MMU_WARN_ON() might change
> numbers. I will run a perf test on your mmu branch and see if
> something changes a lot.

It won't.  MMU_WARN_ON() is dead code without manual modification to define MMU_DEBUG.
Part of the reason I used MMU_WARN_ON() was to remind myself to send a patch/series
to overhaul MMU_WARN_ON[*].  My thought/hope is that a Kconfig will allow developers
and testers to run with a pile of assertions and sanity checks without impacting
the runtime overhead for production builds.

[*] https://lore.kernel.org/all/Yz4Qi7cn7TWTWQjj@google.com/