linux-kernel - Re: [PATCH v2 06/12] KVM: MMU: introduce a static table to map guest access to spte access

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Date:	Tue, 29 Jan 2013 15:16:51 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Avi Kivity <avi.kivity@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 06/12] KVM: MMU: introduce a static table to map guest
 access to spte access

On Mon, Jan 28, 2013 at 11:07:58PM -0200, Marcelo Tosatti wrote:
> On Mon, Jan 28, 2013 at 10:07:15PM -0200, Marcelo Tosatti wrote:
> > On Fri, Jan 25, 2013 at 10:46:31AM +0800, Xiao Guangrong wrote:
> > > On 01/25/2013 08:15 AM, Marcelo Tosatti wrote:
> > > > On Wed, Jan 23, 2013 at 06:07:20PM +0800, Xiao Guangrong wrote:
> > > >> It makes set_spte more clean and reduces branch prediction
> > > >>
> > > >> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> > > >> ---
> > > >>  arch/x86/kvm/mmu.c |   37 ++++++++++++++++++++++++++-----------
> > > >>  1 files changed, 26 insertions(+), 11 deletions(-)
> > > > 
> > > > Don't see set_spte as being a performance problem?
> > > > IMO the current code is quite simple.
> > > 
> > > Yes, this is not a performance problem.
> > > 
> > > I just dislike this many continuous "if"-s in the function:
> > > 
> > > if (xxx)
> > > 	xxx
> > > if (xxx)
> > > 	xxx
> > > ....
> > > 
> > > Totally, it has 7 "if"-s before this patch.
> > > 
> > > Okay, if you think this is unnecessary, i will drop this patch. :)
> > 
> > Yes, please (unless you can show set_spte is a performance problem).
> 
> Same thing for spte fast drop: is it a performance problem? 
> 
I like spte fast drop because it gets rid of "goto restart" pattern.
for_each_spte_in_rmap_safe() can be the alternative.

> Please try to group changes into smaller, less controversial sets with 
> a clear goal:
> 
> - Debated performance improvement.
> - Cleanups (eg mmu_set_spte argument removal).
> - Bug fixes.
> - Performance improvements.
> 
> Thanks.

--
			Gleb.
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