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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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