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

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Date:   Fri, 30 Jul 2021 11:45:53 -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.

Since at it, I'll further move "more" to be at the entry too, so I think it
optimizes full entries case too.

/*
 * Slight optimization of cacheline layout, by putting `more' and `spte_count'
 * at the start; then accessing it will only use one single cacheline for
 * either full (entries==PTE_LIST_EXT) case or entries<=6.
 */
struct pte_list_desc {
	struct pte_list_desc *more;
	/*
	 * 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;
	u64 *sptes[PTE_LIST_EXT];
};

Thanks,

-- 
Peter Xu