[PATCH 08/37] KVM: arm64: Defer restoring host VFP state to vcpu_put

[Christoffer Dall]

Hi Drew,

On Wed, Nov 15, 2017 at 05:04:40PM +0100, Andrew Jones wrote:
> On Thu, Oct 12, 2017 at 12:41:12PM +0200, Christoffer Dall wrote:
> > Avoid saving the guest VFP registers and restoring the host VFP
> > registers on every exit from the VM.  Only when we're about to run
> > userspace or other threads in the kernel do we really have to switch the
> > state back to the host state.
> 
> Rik van Riel's recently post patch "[PATCH v2 0/2] x86,kvm: move qemu/guest
> FPU switching out to kvm_arch_vcpu_ioctl_run" indicates that for x86 they
> only need to swap guest and userspace VFP registers before exiting VCPU_RUN
> to userspace, not for running other threads. I imagine that's the same for
> ARM as well.
> 
> If so, then I think this hunk
> 
> > @@ -209,4 +212,16 @@ void kvm_vcpu_load_sysregs(struct kvm_vcpu *vcpu)
> >   */
> >  void kvm_vcpu_put_sysregs(struct kvm_vcpu *vcpu)
> >  {
> > +	struct kvm_cpu_context *host_ctxt = vcpu->arch.host_cpu_context;
> > +	struct kvm_cpu_context *guest_ctxt = &vcpu->arch.ctxt;
> > +
> > +	/* Restore host FP/SIMD state */
> > +	if (vcpu->arch.guest_vfp_loaded) {
> > +		if (vcpu_el1_is_32bit(vcpu))
> > +			kvm_call_hyp(__fpsimd32_save_state,
> > +				     kern_hyp_va(guest_ctxt));
> > +		__fpsimd_save_state(&guest_ctxt->gp_regs.fp_regs);
> > +		__fpsimd_restore_state(&host_ctxt->gp_regs.fp_regs);
> > +		vcpu->arch.guest_vfp_loaded = 0;
> > +	}
> >  }
> 
> could be moved to the return of kvm_arch_vcpu_ioctl_run().
> 
That sounds cool.

I'll keep this patch as it is now, and look at Rik's patches and post a
follow up later, does that sound ok?

Thanks,
-Christoffer