KVM: arm64/sve: Explain validity checks in set_sve_vls()

Correct virtualization of SVE relies for correctness on code in
set_sve_vls() that verifies consistency between the set of vector
lengths requested by userspace and the set of vector lengths
available on the host.

However, the purpose of this code is not obvious, and not likely to
be apparent at all to people who do not have detailed knowledge of
the SVE system-level architecture.

This patch adds a suitable comment to explain what these checks are
for.

No functional change.

Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/kvm/guest.c

@@ -264,6 +264,13 @@ static int set_sve_vls(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 	if (max_vq > sve_vq_from_vl(kvm_sve_max_vl))
 		return -EINVAL;
 
+	/*
+	 * Vector lengths supported by the host can't currently be
+	 * hidden from the guest individually: instead we can only set a
+	 * maxmium via ZCR_EL2.LEN.  So, make sure the available vector
+	 * lengths match the set requested exactly up to the requested
+	 * maximum:
+	 */
 	for (vq = SVE_VQ_MIN; vq <= max_vq; ++vq)
 		if (vq_present(&vqs, vq) != sve_vq_available(vq))
 			return -EINVAL;