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

[Christoffer Dall <christoffer.dall@linaro.org>]

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...

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

Thanks,
-Christoffer