Re: [RFC PATCH] arm64: KVM: remove fpsimd save/restore from the world switch

Christoffer Dall
2015-04-10T09:04:37Z

On Thu, Apr 09, 2015 at 05:53:59PM +0100, Marc Zyngier wrote:
> The world switch spends quite some time dealing with the FP/SIMD
> registers, as the state is quite sizeable (32 128bit registers,
> plus some crumbs on the side). We save/restore them on each
> entry/exit, so that both the host and the guest always see
> the state they expect.
>
> But let's face it: the host kernel doesn't care. It is the host
> userspace that actually cares about FP. An obvious improvement is
> to remove the save/restore from the world switch, and only perform
> it when we're about to enter/exit the guest (by plugging it into
> vcpu_load/vcpu_put). The effect is pretty spectacular when running
> hackbench (which is the only benchmark worth looking at):
>
so the kernel never uses fp/simd registers for stuff like memcopies
etc.?

Can we also make a similar change for ARM on the 32-bit side?

-Christoffer