Re: Paravirt KVM capabilities

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Ingo Molnar wrote:
> * Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
>
>   
>> For i386 Xen does not switch cr3 IIRC. [...]
>>     
>
> correct.
>
>   
>> [...] Perhaps even not for x86_64 if it can use the segment limits 
>> which AMD re-added (I think it does?)
>>     
>
> i'm not sure. Older ones definitely used cr3 switching and on Intel 
> there's no segment limits on 64-bit AFAIK.
>
>   

I'd be surprised if that remained the case for too long.


>> I think for i386 Xen does not go through the hypervisor at all: it 
>> hacks int 0x80 to trap directly to ring 1.  So there's still the 
>> overhead of using int rather than sysenter, but not much more. [...]
>>     
>
> that alone is already ~100-200 cycles overhead, it essentially doubles 
> the null syscall overhead. It matters at millions of syscalls per second 
> workloads, 100 cycles overhead at 1 million syscalls a second means 5% 
> performance difference at 2GHz.
>
>   

That doesn't strike me as too bad.


> and longer-term, the cost of VM exit due to IRQs is expected to get 
> cheaper. The cost of int $0x80 versus SYSENTER will stay constant, or 
> will get worse.
>   

Agreed.  But I still don't know whether the lines on the graph will cross.


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


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