Re: [PATCH 09/10] KVM: arm64: Save/restore SVE state for nVHE

[Will Deacon <will@kernel.org>]

On Tue, Mar 16, 2021 at 10:13:11AM +0000, Marc Zyngier wrote:
> Implement the SVE save/restore for nVHE, following a similar
> logic to that of the VHE implementation:
> 
> - the SVE state is switched on trap from EL1 to EL2
> 
> - no further changes to ZCR_EL2 occur as long as the guest isn't
>   preempted or exit to userspace
> 
> - on vcpu_put(), ZCR_EL2 is reset to its default value, and ZCR_EL1
>   restored to the default guest value
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/fpsimd.c                 | 15 ++++++++--
>  arch/arm64/kvm/hyp/include/hyp/switch.h | 37 +++++++++----------------
>  arch/arm64/kvm/hyp/nvhe/switch.c        |  4 +--
>  3 files changed, 28 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
> index b5f95abd23f5..cc6cdea69596 100644
> --- a/arch/arm64/kvm/fpsimd.c
> +++ b/arch/arm64/kvm/fpsimd.c
> @@ -121,11 +121,22 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
>  	local_irq_save(flags);
>  
>  	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
> -		if (guest_has_sve)
> +		if (guest_has_sve) {
>  			__vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_el1(SYS_ZCR);
>  
> +			/* Restore the VL that was saved when bound to the CPU */
> +			if (!has_vhe()) {
> +				u64 zcr;
> +
> +				kvm_call_hyp(__kvm_reset_sve_vq);

What would go wrong if we did this unconditionally on return to the host, or
is it just a performance thing to move work off the fast path where we
return back to the same vCPU?

Will