Re: Host latency peaks due to kvm-intel

[Jan Kiszka <jan.kiszka@web.de>]

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Avi Kivity wrote:
> On 07/25/2009 12:55 PM, Jan Kiszka wrote:
>> Avi Kivity wrote:
>>   
>>> On 07/24/2009 12:41 PM, Jan Kiszka wrote:
>>>     
>>>> I vaguely recall that someone promised to add a feature reporting
>>>> facility for all those nice things, modern VM-extensions may or may not
>>>> support (something like or even an extension of /proc/cpuinfo). What is
>>>> the state of this plan? Would be specifically interesting for Intel
>>>> CPUs
>>>> as there seem to be many of them out there with restrictions for
>>>> special
>>>> use cases - like real-time.
>>>>
>>>>        
>>> Newer kernels do report some vmx features (like flexpriority) in
>>> /proc/cpuinfo but not all.
>>>
>>>      
>>
>> Ah, nice. Then we just need this?
>>
>> ------------>
>>
>> From: Jan Kiszka<jan.kiszka@siemens.com>
>> Subject: [PATCH] x86: Report VMX feature vwbinvd
>>
>> Not all VMX-capable CPUs support guest exists on wbinvd execution. If
>> this is not supported, the instruction will run natively on behalf of
>> the guest. This can cause multi-millisecond latencies to the host which
>> is very problematic in real-time scenarios.
>>
>> Report the wbinvd trapping feature along with other VMX feature flags,
>> calling it 'vwbinvd' ('virtual wbinvd').
>>
>>    
> 
> What about AMD cpus that can always trap wbinvd?  do we set the bit or
> do we trust the user to know that it isn't needed on AMD (I suppose the
> latter)?

I also think that the feature flags should remain vendor-specific.

> 
> This should go in via tip.git, it isn't really kvm related (except that
> kvm should start reading these caps one day instead of querying the
> hardware directly).
> 

OK, will go that way. Probably I will also add some flags for AMD's NPT,
Intel's EPT and they new unrestricted guest mode at this chance.

Jan


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