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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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