linux-kernel - Re: [Patch v2 2/5] KVM: x86/mmu: Optimize SPTE change flow for clear-dirty-log

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Date:   Wed, 8 Feb 2023 15:44:45 -0800
From:   Vipin Sharma <vipinsh@...gle.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     seanjc@...gle.com, pbonzini@...hat.com, bgardon@...gle.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch v2 2/5] KVM: x86/mmu: Optimize SPTE change flow for clear-dirty-log

On Tue, Feb 7, 2023 at 9:47 AM David Matlack <dmatlack@...gle.com> wrote:
>
> On Tue, Feb 7, 2023 at 9:37 AM Vipin Sharma <vipinsh@...gle.com> wrote:
> >
> > On Mon, Feb 6, 2023 at 3:41 PM David Matlack <dmatlack@...gle.com> wrote:
> > >
> > > On Fri, Feb 03, 2023 at 11:28:19AM -0800, Vipin Sharma wrote:
> > >
> > >         if (wrprot || spte_ad_need_write_protect(iter.old_spte))
> > >                 clear_bits = PT_WRITABLE_MASK;
> > >         else
> > >                 clear_bits = shadow_dirty_mask;
> > >
> > >         if (!(iter->old_spte & clear_bits))
> > >                 continue;
> > >
> > >         iter.old_spte = kvm_tdp_mmu_clear_spte_bit(&iter, clear_bits);
> > >
> >
> > Yeah, this is better. Even better if I just initialize like:
> >
> > u64 clear_bits = shadow_dirty_mask;
> >
> > This will also get rid of the else part.
>
> On that topic... Do we need to recalculate clear_bits for every spte?
> wrprot is passed as a parameter so that will not change. And
> spte_ad_need_write_protect() should either return true or false for
> all SPTEs in the TDP MMU. Specifically, make_spte() has this code:
>
> if (sp->role.ad_disabled)
>         spte |= SPTE_TDP_AD_DISABLED_MASK;
> else if (kvm_mmu_page_ad_need_write_protect(sp))
>         spte |= SPTE_TDP_AD_WRPROT_ONLY_MASK;
>
> sp->role.ad_disabled is never modified in TDP MMU pages. So it should
> be constant for all pages. And kvm_mmu_page_ad_need_write_protect()
> will always return false for TDP MMU pages since sp->role.guest_mode
> is always false.
>
> So this can just be:
>
> u64 clear_bit = (wrprot || !kvm_ad_enabled()) ? PT_WRITABLE_MASK :
> shadow_dirty_mask;

I discussed it offline with David to understand more about it. It
makes sense as TDP MMU pages will not have nested SPTEs (they are in
rmaps). So, this looks good, I will add it in the next version.

Thanks