linux-kernel - Re: [RFC] syscall calling convention, stts/clts, and xstate latency

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Message-ID: <4E2D1E57.1080404@redhat.com>
Date:	Mon, 25 Jul 2011 10:42:15 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Andrew Lutomirski <luto@....edu>, linux-kernel@...r.kernel.org,
	x86 <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC] syscall calling convention, stts/clts, and xstate latency

On 07/25/2011 12:15 AM, Ingo Molnar wrote:
> >  All of this makes me think that, at least on Sandy Bridge, lazy
> >  xstate saving is a bad optimization -- if the cache is being nice,
> >  save/restore is faster than twiddling the TS bit.  And the cost of
> >  the trap when TS is set blows everything else away.
>
> Interesting. Mind cooking up a delazying patch and measure it on
> native as well? KVM generally makes exceptions more expensive, so the
> effect of lazy exceptions might be less on native.

While this is true in general, kvm will trap #NM only after a host 
context switch or an exit to host userspace.  These are supposedly rare 
so you won't see them a lot, especially in a benchmark scenario with 
just one guest.

("host context switch" includes switching to the idle thread when the 
guest executes HLT, something I tried to optimize in the past but it 
proved too difficult for the gain)

-- 
error compiling committee.c: too many arguments to function

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