Re: State of current pv_ops backend for KVM?

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> Avi Kivity wrote:
>>> Anthony Liguori wrote:
>>>> Howdy,
>>>>
>>>> Does anyone know what the state of a pv_ops backend for KVM is?  I 
>>>> know Ingo has an implementation that implements CR3 caching but I 
>>>> don't see any branches in Avi's git tree.
>>>>
>>>> Perhaps we should try for a simple pv_ops backend for 2.6.23 seeing 
>>>> as how the host infrastructure is there?  I'd be willing to do some 
>>>> leg work here... 
>>>
>>> I think we should aim for a full direct paging implementation, where 
>>> the cr3 cache is just the cherry on top that makes context switches 
>>> proceed at native speed.  Jun has a paper on it for OLS and I hope 
>>> patches too.
>>
>> That's a bit more ambitious than I was thinking.  I would be very 
>> happy with a pv timer, interrupt controller, and some basic CPU 
>> optimizations.  I'm still not convinced the perf benefits of direct 
>> paging justify the annoyances that come with it (guests being aware 
>> of pfns).  
>
> I thought that with paravirt_ops, all the awareness is limited to the 
> paravirt implementation, and nothing in the core kernel is actually 
> aware of host pfns.

The pv_ops implementation still has to deal with guest pfn => host pfn 
translation.  This means that a guest has to be put into a special mode 
for save/restore/migrate to work to ensure that no host pfn's are on the 
stack or in registers.  Changing save/restore/migration to from 
guest-transparent to guest-interactive is a big loss to me.

> pv timer and interrupt controller make a lot of sense, especially if 
> steal timer functionality is included.

Yup :-)

>> This is especially true with NPT/EPT right around the corner.
>
> It depends on the workload.  It was advanced (by Ingo) that NPT/EPT 
> increase the tlb miss latency, which is important for steady-state 
> workloads, whereas shadow and direct paging do not.  It will be 
> interesting to measure this.

There will most certainly be trade-offs, but I certainly don't expect 
direct paging to beat out NPT/EPT in certain areas.

Regards,

Anthony Liguori


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