Re: [PATCH 0/3] arm: KVM: VFP lazy switch in KVM Host Mode may save upto 98%

[Mario Smarduch <m.smarduch@samsung.com>]

On 07/05/2015 12:37 PM, Christoffer Dall wrote:
> Hi Mario,
> 
> On Wed, Jun 24, 2015 at 08:30:25PM -0700, Mario Smarduch wrote:
>> Currently we do a lazy VFP switch in Hyp mode, but once we exit and re-enter hyp
>> mode we trap again on VFP access. This mode has shown around 30-50% improvement
>> running hackbench and lmbench.
>>
>> This patch series extends lazy VFP switch beyond Hyp mode to KVM host mode.
>>
>> 1 - On guest access we switch from host to guest and set a flag accessible to 
>>     host
>> 2 - On exit to KVM host, VFP state is restored on vcpu_put if flag is marked (1)
>> 3 - Otherwise guest is resumed and continues to use its VFP registers. 
>> 4 - In case of 2 on VM entry we set VFP trap flag to repeat 1.
>>
>> If guest does not access VFP registers them implemenation remains the same.
>>
>> Executing hackbench on Fast Models and Exynos arm32 board shows good
>> results. Considering all exits 2% of the time KVM host lazy vfp switch is 
>> invoked.
>>
>> Howeverr this patch set requires more burn in time and testing under various 
>> loads.
>>
>> Currently ARM32 is addressed later ARM64.
>>
> I think Marc said that he experimented with a similar patch once, but
> that it caused corruption on the host side somehow.
> 
> Am I remembering correctly?  If so, we would need to make sure this
> doesn't happen with this patch set...

I think upstreaming the basic approach first (arm64, arm32 cleanups),
and let this series get some good runtime - would be better
and safer approach.


> 
> Otherwise I think this sounds like a fairly good idea and I wonder if
> the same could be done on arm64?

Yes that's the intent, doing both architectures at once would be preferable.

Thanks,
  Mario
> 
> Thanks,
> -Christoffer
>