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

[Andrew Jones]

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().

Thanks,
drew