KVM: arm64: nv: Ensure correct VL is loaded before saving SVE state

It is possible that the guest hypervisor has selected a smaller VL than
the maximum for its nested guest. As such, ZCR_EL2 may be configured for
a different VL when exiting a nested guest.

Set ZCR_EL2 (via the EL1 alias) to the maximum VL for the VM before
saving SVE state as the SVE save area is dimensioned by the max VL.

Reviewed-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Oliver Upton <[email protected]>

Author: oupton

arch/arm64/kvm/fpsimd.c

@@ -195,11 +195,14 @@ void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu)
 			 * Note that this means that at guest exit ZCR_EL1 is
 			 * not necessarily the same as on guest entry.
 			 *
-			 * Restoring the VL isn't needed in VHE mode since
-			 * ZCR_EL2 (accessed via ZCR_EL1) would fulfill the same
-			 * role when doing the save from EL2.
+			 * ZCR_EL2 holds the guest hypervisor's VL when running
+			 * a nested guest, which could be smaller than the
+			 * max for the vCPU. Similar to above, we first need to
+			 * switch to a VL consistent with the layout of the
+			 * vCPU's SVE state. KVM support for NV implies VHE, so
+			 * using the ZCR_EL1 alias is safe.
 			 */
-			if (!has_vhe())
+			if (!has_vhe() || (vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu)))
 				sve_cond_update_zcr_vq(vcpu_sve_max_vq(vcpu) - 1,
 						       SYS_ZCR_EL1);
 		}