Re: [RFT] mmu optimizations branch

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@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.
>   

Does this really call for parvirtualization? Caching the old cr3 value 
will work for native as well.


-- 
error compiling committee.c: too many arguments to function


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