KVM: arm64: Use get_raz_reg() for userspace reads of PMSWINC_EL0

PMSWINC_EL0 is a write-only register and was initially part of the VCPU
register state, but was later removed in commit 7a3ba3095a32 ("KVM:
arm64: Remove PMSWINC_EL0 shadow register"). To prevent regressions, the
register was kept accessible from userspace as Read-As-Zero (RAZ).

The read function that is used to handle userspace reads of this
register is get_raz_id_reg(), which, while technically correct, as it
returns 0, it is not semantically correct, as PMSWINC_EL0 is not an ID
register as the function name suggests.

Add a new function, get_raz_reg(), to use it as the accessor for
PMSWINC_EL0, as to not conflate get_raz_id_reg() to handle other types
of registers.

No functional change intended.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211011105840.155815-3-alexandru.elisei@arm.com

arch/arm64/kvm/sys_regs.c

@@ -1285,6 +1285,15 @@ static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 	return __set_id_reg(vcpu, rd, uaddr, true);
 }
 
+static int get_raz_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
+		       const struct kvm_one_reg *reg, void __user *uaddr)
+{
+	const u64 id = sys_reg_to_index(rd);
+	const u64 val = 0;
+
+	return reg_to_user(uaddr, &val, id);
+}
+
 static int set_wi_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 		      const struct kvm_one_reg *reg, void __user *uaddr)
 {
@@ -1647,7 +1656,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	 * previously (and pointlessly) advertised in the past...
 	 */
 	{ PMU_SYS_REG(SYS_PMSWINC_EL0),
-	  .get_user = get_raz_id_reg, .set_user = set_wi_reg,
+	  .get_user = get_raz_reg, .set_user = set_wi_reg,
 	  .access = access_pmswinc, .reset = NULL },
 	{ PMU_SYS_REG(SYS_PMSELR_EL0),
 	  .access = access_pmselr, .reset = reset_pmselr, .reg = PMSELR_EL0 },