KVM: arm64: Fix host's ZCR_EL2 restore on nVHE

We re-enter the EL1 host with CPTR_EL2.TZ set in order to
be able to lazily restore ZCR_EL2 when required.

However, the same CPTR_EL2 configuration also leads to trapping
when ZCR_EL2 is accessed from EL2. Duh!

Clear CPTR_EL2.TZ *before* writing to ZCR_EL2.

Fixes: beed0906 ("KVM: arm64: Trap host SVE accesses when the FPSIMD state is dirty")
Reported-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/hyp/nvhe/hyp-main.c

@@ -178,8 +178,9 @@ void handle_trap(struct kvm_cpu_context *host_ctxt)
 		handle_host_smc(host_ctxt);
 		break;
 	case ESR_ELx_EC_SVE:
-		sve_cond_update_zcr_vq(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
 		sysreg_clear_set(cptr_el2, CPTR_EL2_TZ, 0);
+		isb();
+		sve_cond_update_zcr_vq(ZCR_ELx_LEN_MASK, SYS_ZCR_EL2);
 		break;
 	default:
 		hyp_panic();