Re: [PATCH 07/10] KVM: arm64: Save guest's ZCR_EL1 before saving the FPSIMD state

[Will Deacon <will@kernel.org>]

On Tue, Mar 16, 2021 at 10:13:09AM +0000, Marc Zyngier wrote:
> Make sure the guest's ZCR_EL1 is saved before we save/flush the
> state. This will be useful in later patches.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/fpsimd.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
> index 84afca5ed6f2..b5f95abd23f5 100644
> --- a/arch/arm64/kvm/fpsimd.c
> +++ b/arch/arm64/kvm/fpsimd.c
> @@ -121,10 +121,10 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
>  	local_irq_save(flags);
>  
>  	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
> -		fpsimd_save_and_flush_cpu_state();
> -
>  		if (guest_has_sve)
>  			__vcpu_sys_reg(vcpu, ZCR_EL1) = read_sysreg_el1(SYS_ZCR);
> +
> +		fpsimd_save_and_flush_cpu_state();

I _think_ fpsimd_save_and_flush_cpu_state() contains a RDVL instruction
to get at the vector length for sve_get_vl(), and this ends up reading from
ZCR_EL1. So I'm not sure it's save to move it like this.

Will