linux-kernel - Re: [PATCH v2 8/9] KVM: X86: Optimize pte_list

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Message-ID: <YQHRV/uEZ4LqPVNI@t490s>
Date:   Wed, 28 Jul 2021 17:51:19 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 8/9] KVM: X86: Optimize pte_list_desc with per-array
 counter

On Wed, Jul 28, 2021 at 09:04:30PM +0000, Sean Christopherson wrote:
> >  struct pte_list_desc {
> >  	u64 *sptes[PTE_LIST_EXT];
> > +	/*
> > +	 * Stores number of entries stored in the pte_list_desc.  No need to be
> > +	 * u64 but just for easier alignment.  When PTE_LIST_EXT, means full.
> > +	 */
> > +	u64 spte_count;
> 
> Per my feedback to the previous patch, this should be above sptes[] so that rmaps
> with <8 SPTEs only touch one cache line.  No idea if it actually matters in
> practice, but I can't see how it would harm anything.

Reasonable.  Not sure whether this would change the numbers a bit in the commit
message; it can be slightly better but also possible to be non-observable.
Paolo, let me know if you want me to repost/retest with the change (along with
keeping the comment in the other patch).

Thanks for looking!

-- 
Peter Xu