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SubjectRe: [RFC] syscall calling convention, stts/clts, and xstate latency

* Avi Kivity <avi@redhat.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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