Re: [PATCH 0/6] KVM: MMU: support pte prefetch when intercepted guest #PF

Avi Kivity
2010-06-15T11:45:08Z

On 06/15/2010 05:46 AM, Xiao Guangrong wrote:
> Hi Avi, Marcelo,
>
> This patchset support pte prefetch when intercepted guest #PF,
> the aim is to reduce guest #PF which can be intercepted by VMM.
>
> If we meet any failure in the prefetch path, we will exit it
> and not try other ptes to avoid become heavy path.
>
> During my performance test, under EPT enabled case, unixbench
> shows the performance improved ~1.2%,

Once the guest has faulted in all memory, we shouldn't see much
improvement, yes?

> user EPT disable case,
> unixbench shows the performance improved ~3.6%
>

I'm a little worried about this. In some workloads, prefetch can often
fail due to gpte.a=0 so we spend effort doing nothing. There is also
the issue of marking pages as accessed or even dirty when in fact the
guest did not access them.

We should map those pages with pte.a=pte.d=0 so we don't confuse host
memory management. On EPT (which lacks a/d bits) we can't enable it
(but we can on NPT).

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