Re: [RFT] mmu optimizations branch

Avi Kivity
2007-01-03T10:16:07Z

Ingo Molnar wrote:
> * Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
>
>
>>> actually, the VM entry+exit cost on this CPU is around 3-4
>>> microseconds, so it's still 2 VM exits per context switch.
>>>
>>> I debugged this a bit, and what happens is that when Linux does a
>>> task-switch it does a cr3 load /and/ a write (look at __flush_tlb())
>>> - and both are causing a vm exit!
>>>
>>> I have started paravirtualizing the Linux kernel for KVM. I have
>>> eliminated the cr3 load from the Linux kernel via paravirtualization
>>> and that way lat_ctx shows a ~5-6 usecs context-switch cost. That's
>>> pretty good i think, compared to the 2-3 usecs of native. I'll send
>>> patches for this tomorrow.
>>>
>> Does this really call for parvirtualization? Caching the old cr3 value
>> will work for native as well.
>>
>
> it's already cached (and my changes use that cached value), but it (used
> to be) faster/smaller to just move from cr3 than to dereference down to
> the cached pgd pointer, because we used to inline those instructions
> heavily. But i agree that in the current kernel it's probably worth
> doing this for native too - i'll cook up a patch for upstream.
>
>

Ok.

A question. What's __flush_tlb() doing in the context switch path?
Shouldn't it just load the new cr3 and be done with it?