Re: High vm-exit latencies during kvm boot-up/shutdown

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

Jan Kiszka wrote:
> Avi Kivity wrote:
>   
>> Jan Kiszka wrote:
>>     
>>> Avi Kivity wrote:
>>>   
>>>       
>>>> Jan Kiszka wrote:
>>>>     
>>>>         
>>>>> I ran
>>>>>
>>>>> user/kvmctl user/test/bootstrap user/test/smp.flat
>>>>>
>>>>> with the busy loop hacked into bootstrap, but I got no latency spots
>>>>> this time. And what should I look for in the output of kvm_stat?
>>>>>
>>>>>   
>>>>>       
>>>>>           
>>>> The first numeric column is the total number of exits; the second is the 
>>>> rate of change (per second).  For a spinning workload, both irq_exits 
>>>> and exits rate should equal the timer frequency.
>>>>     
>>>>         
>>> It matches roughly, but this isn't surprising given the lack of latency
>>> peaks in this scenario.
>>>
>>>   
>>>       
>> Okay, that's expected.  But I really can't think what causes the latency 
>> spike you see running the bios code. 300 microseconds is enough for 3000 
>> cache misses.  No instruction can cause so many except rep movs and 
>> friends, and these are supposed to be preemptible.
>>
>> You might try isolating it by adding code to output markers to some I/O 
>> port and scattering it throughout the bios.  A binary search should find 
>> the problem spot quickly.
>>
>>     
>
> Got it! It's wbinvd from smm_init in rombios32.c! Anyone any comments on
> this?
>   

Ha!  A real life 300usec instruction!

Unfortunately, it cannot be trapped on Intel (it can be trapped on 
AMD).  Looks like a minor hole in VT, as a guest can generate very high 
latencies this way.

For our bios, we can remove it, but there's no way to ensure an 
untrusted guest doesn't use it.

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


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