linux-kernel - Re: [PATCH 3/6] KVM MMU: optimize/cleanup for marking parent unsync

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Date:	Wed, 14 Apr 2010 13:35:44 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, KVM list <kvm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/6] KVM MMU: optimize/cleanup for marking parent unsync

On Wed, Apr 14, 2010 at 11:23:38AM +0800, Xiao Guangrong wrote:
> 
> 
> Marcelo Tosatti wrote:
> 
> >>> I'd prefer to not touch it.
> >> This patch avoids walk all parents and i think this overload is really unnecessary.
> >> It has other tricks in this codepath but i not noticed? :-)
> > 
> > My point is that there is no point in optimizing something unless its
> > performance sensitive.
> 
> Hi Marcelo,
> 
> I think optimizing not only means 'performance' but also means 'smaller code'(maybe 'cleanup'
> is more suitable) and 'logic optimize'(do little things), i'm not sure this patch whether can
> improve system performance obviously but it optimize the code logic and reduce code size, and
> it not harm other code and system performance, right? :-)

Right, but this walking code already is compact and stable. Removing the
unused code variables/definitions is fine, but i'd prefer to not change
the logic just for the sake of code reduction.

> Actually, the origin code has a bug, the code segment in mmu_parent_walk():
> 
> |	if (!sp->multimapped && sp->parent_pte) {
> |		......
> |		return;
> |	}
> |	hlist_for_each_entry(pte_chain, node, &sp->parent_ptes, link)
> |		for (i = 0; i < NR_PTE_CHAIN_ENTRIES; ++i) {
> |			......
> |		}
> 
> So, if sp->parent_pte == NULL, it's unsafe...
> 
> > And as i recall, mmu_unsync_walk was much more
> > sensitive performance wise than parent walking. Actually, gfn_to_memslot 
> > seems more important since its also noticeable on EPT/NPT hosts.
> 
> Yeah, i also noticed these and i'm looking into these code.

Great.

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