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

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Message-ID: <20110725075422.GB32294@elte.hu>
Date:	Mon, 25 Jul 2011 09:54:22 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
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


* Avi Kivity <avi@...hat.com> wrote:

> 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)

Yeah - but this was a fair thing to test before Andy embarks on 
something more ambitious on the native side.

Thanks,

	Ingo
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