Re: [PATCH 1/2] x86,kvm: move qemu/guest FPU switching out to vcpu_run

[Rik van Riel]

On Wed, 2017-11-15 at 08:47 +0800, Wanpeng Li wrote:
> 2017-11-15 5:54 GMT+08:00ÂÂ<riel@xxxxxxxxxx>:
> > From: Rik van Riel <riel@xxxxxxxxxx>
> > 
> > Currently, every time a VCPU is scheduled out, the host kernel will
> > first save the guest FPU/xstate context, then load the qemu
> > userspace
> > FPU context, only to then immediately save the qemu userspace FPU
> > context back to memory. When scheduling in a VCPU, the same
> > extraneous
> > FPU loads and saves are done.
> > 
> > This could be avoided by moving from a model where the guest FPU is
> > loaded and stored with preemption disabled, to a model where the
> > qemu userspace FPU is swapped out for the guest FPU context for
> > the duration of the KVM_RUN ioctl.
> 
> What will happen if CONFIG_PREEMPT is enabled?

The scheduler will save the guest FPU context when a
VCPU thread is preempted, and restore it when it is
scheduled back in.

While inside kvm_arch_vcpu_ioctl_run, the task FPU
context will be the guest FPU context, instead of the
qemu userspace FPU context.