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

Avi Kivity
2010-06-17T08:00:36Z

On 06/17/2010 10:49 AM, Xiao Guangrong wrote:
>
> Avi Kivity wrote:
>
>> 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?
>>
> I think you are right, this path only prefetch valid/pte.A=1 mapping.
>

I mean for tdp. Faulting is rare once the guest has touched all of memory.

>>> 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.
>>
> Yes, prefetch is not alway success, but the prefetch path is fast, it not cost
> much time, at the worst case, only 128 bytes we need read form guest pte. Once
> it's successful, much overload can be reduce.
>

Ok.

>> 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).
>>
>>
> You are right, this is the speculative path.
>
> For the pte.A bit:
> we called mmu_set_spte() with speculative = true, so we set pte.a = 0 in this
> path.
>
> For the pte.D bit:
> We should fix also set pte.d = 0 in speculative path, the same problem is in
> invlpg/pte write path, will do it in the next version.
>

It's not enough to set spte.d=0, we also need to sample it later.

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