Re: [RFT] mmu optimizations branch

Anthony Liguori
2007-01-03T03:05:46Z

Ingo Molnar wrote:
> * Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
>
>
>>> lat_ctx -s 0 [zero memory footprint]:
>>>
>>> -------------------------------------------------
>>> #tasks native kvm-r4204 kvm-r4232(mmu)
>>> -------------------------------------------------
>>> 2: 2.02 180.91 9.19
>>> 20: 4.04 183.21 10.01
>>> 50: 4.30 185.95 11.27
>>>
>>> so here it's a /massive/, almost 20 times speedup!
>>>
>>>
>> Excellent. 10us is approximately the vmexit overhead on intel (we
>> regularly see 100-120k exits/sec), so it means a context switch is
>> exactly one exit. Hard to beat without nested page tables.
>>
>
> 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.
>

This should be hookable via arch_{enter,leave}_cpu_mode() via
paravirt_ops. I was actually just looking at this myself (although I
was focusing on lazy mmu hooks). I've taken the route of using a VMI
ROM to actually hook it (instead of implementing a custom paravirt_ops
for KVM).

I can post the ROM if you're interested in this sort of approach. I
like using VMI as we can access most of the things hookable by
paravirt_ops without having to change a kernel binary.

Regards,

Anthony Liguori

> Ingo
>
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