KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

KVM does not yet support userspace modifying PMCR_EL0.N (With
the previous patch, KVM ignores what is written by userspace).
Add support userspace limiting PMCR_EL0.N.

Disallow userspace to set PMCR_EL0.N to a value that is greater
than the host value as KVM doesn't support more event counters
than what the host HW implements. Also, make this register
immutable after the VM has started running. To maintain the
existing expectations, instead of returning an error, KVM
returns a success for these two cases.

Finally, ignore writes to read-only bits that are cleared on
vCPU reset, and RES{0,1} bits (including writable bits that
KVM doesn't support yet), as those bits shouldn't be modified
(at least with the current KVM).
Co-developed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Link: https://lore.kernel.org/r/20231020214053.2144305-8-rananta@google.comSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/sys_regs.c

@@ -719,9 +719,9 @@ static unsigned int pmu_visibility(const struct kvm_vcpu *vcpu,
 
 static u64 reset_pmu_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 {
-	u64 n, mask = BIT(ARMV8_PMU_CYCLE_IDX);
+	u64 mask = BIT(ARMV8_PMU_CYCLE_IDX);
+	u8 n = vcpu->kvm->arch.pmcr_n;
 
-	n = vcpu->kvm->arch.pmcr_n;
 	if (n)
 		mask |= GENMASK(n - 1, 0);
 
@@ -1136,6 +1136,44 @@ static int get_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
 	return 0;
 }
 
+static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r,
+		    u64 val)
+{
+	u8 new_n = (val >> ARMV8_PMU_PMCR_N_SHIFT) & ARMV8_PMU_PMCR_N_MASK;
+	struct kvm *kvm = vcpu->kvm;
+
+	mutex_lock(&kvm->arch.config_lock);
+
+	/*
+	 * The vCPU can't have more counters than the PMU hardware
+	 * implements. Ignore this error to maintain compatibility
+	 * with the existing KVM behavior.
+	 */
+	if (!kvm_vm_has_ran_once(kvm) &&
+	    new_n <= kvm_arm_pmu_get_max_counters(kvm))
+		kvm->arch.pmcr_n = new_n;
+
+	mutex_unlock(&kvm->arch.config_lock);
+
+	/*
+	 * Ignore writes to RES0 bits, read only bits that are cleared on
+	 * vCPU reset, and writable bits that KVM doesn't support yet.
+	 * (i.e. only PMCR.N and bits [7:0] are mutable from userspace)
+	 * The LP bit is RES0 when FEAT_PMUv3p5 is not supported on the vCPU.
+	 * But, we leave the bit as it is here, as the vCPU's PMUver might
+	 * be changed later (NOTE: the bit will be cleared on first vCPU run
+	 * if necessary).
+	 */
+	val &= ARMV8_PMU_PMCR_MASK;
+
+	/* The LC bit is RES1 when AArch32 is not supported */
+	if (!kvm_supports_32bit_el0())
+		val |= ARMV8_PMU_PMCR_LC;
+
+	__vcpu_sys_reg(vcpu, r->reg) = val;
+	return 0;
+}
+
 /* Silly macro to expand the DBG{BCR,BVR,WVR,WCR}n_EL1 registers in one go */
 #define DBG_BCR_BVR_WCR_WVR_EL1(n)					\
 	{ SYS_DESC(SYS_DBGBVRn_EL1(n)),					\
@@ -2201,8 +2239,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	{ SYS_DESC(SYS_CTR_EL0), access_ctr },
 	{ SYS_DESC(SYS_SVCR), undef_access },
 
-	{ PMU_SYS_REG(PMCR_EL0), .access = access_pmcr,
-	  .reset = reset_pmcr, .reg = PMCR_EL0, .get_user = get_pmcr },
+	{ PMU_SYS_REG(PMCR_EL0), .access = access_pmcr, .reset = reset_pmcr,
+	  .reg = PMCR_EL0, .get_user = get_pmcr, .set_user = set_pmcr },
 	{ PMU_SYS_REG(PMCNTENSET_EL0),
 	  .access = access_pmcnten, .reg = PMCNTENSET_EL0,
 	  .get_user = get_pmreg, .set_user = set_pmreg },