Re: [RFT] mmu optimizations branch

[Anthony Liguori <aliguori-NZpS4cJIG2HvQtjrzfazuQ@public.gmane.org>]

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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