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

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Message-ID: <20100413150137.GB24128@amt.cnet>
Date:	Tue, 13 Apr 2010 12:01:37 -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 Tue, Apr 13, 2010 at 09:53:07AM +0800, Xiao Guangrong wrote:
> 
> 
> Marcelo Tosatti wrote:
> 
> > Xiao,
> > 
> > Did you actually see this codepath as being performance sensitive? 
> 
> Actually, i not run benchmarks to contrast the performance before this patch
> and after this patch.
> 
> > 
> > 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. 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.

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