KVM: arm64: PMU: Simplify PMCR_EL0 reset handling

Resetting PMCR_EL0 is a pretty involved process that includes
poisoning some of the writable bits, just because we can.

It makes it hard to reason about about what gets configured,
and just resetting things to 0 seems like a much saner option.

Reduce reset_pmcr() to just preserving PMCR_EL0.N from the host,
and setting PMCR_EL0.LC if we don't support AArch32.
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/sys_regs.c

@@ -639,24 +639,18 @@ static void reset_pmselr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 
 static void reset_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
-	u64 pmcr, val;
+	u64 pmcr;
 
 	/* No PMU available, PMCR_EL0 may UNDEF... */
 	if (!kvm_arm_support_pmu_v3())
 		return;
 
-	pmcr = read_sysreg(pmcr_el0);
-	/*
-	 * Writable bits of PMCR_EL0 (ARMV8_PMU_PMCR_MASK) are reset to UNKNOWN
-	 * except PMCR.E resetting to zero.
-	 */
-	val = ((pmcr & ~ARMV8_PMU_PMCR_MASK)
-	       | (ARMV8_PMU_PMCR_MASK & 0xdecafbad)) & (~ARMV8_PMU_PMCR_E);
+	/* Only preserve PMCR_EL0.N, and reset the rest to 0 */
+	pmcr = read_sysreg(pmcr_el0) & ARMV8_PMU_PMCR_N_MASK;
 	if (!kvm_supports_32bit_el0())
-		val |= ARMV8_PMU_PMCR_LC;
-	if (!kvm_pmu_is_3p5(vcpu))
-		val &= ~ARMV8_PMU_PMCR_LP;
-	__vcpu_sys_reg(vcpu, r->reg) = val;
+		pmcr |= ARMV8_PMU_PMCR_LC;
+
+	__vcpu_sys_reg(vcpu, r->reg) = pmcr;
 }
 
 static bool check_pmu_access_disabled(struct kvm_vcpu *vcpu, u64 flags)