[PATCH v4 10/14] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest

[Reiji Watanabe]

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

Disallow userspace to set PMCR_EL0.N to a value that is greater
than the host value (KVM_SET_ONE_REG will fail), as KVM doesn't
support more event counters than the host HW implements.
Although this is an ABI change, this change only affects
userspace setting PMCR_EL0.N to a larger value than the host.
As accesses to unadvertised event counters indices is CONSTRAINED
UNPREDICTABLE behavior, and PMCR_EL0.N was reset to the host value
on every vCPU reset before this series, I can't think of any
use case where a user space would do that.

Also, 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).

Signed-off-by: Reiji Watanabe <reijiw at google.com>
---
 arch/arm64/include/asm/kvm_host.h |  3 ++
 arch/arm64/kvm/pmu-emul.c         |  1 +
 arch/arm64/kvm/sys_regs.c         | 48 ++++++++++++++++++++++++++++++-
 3 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 734f1b6f7468..cd0014d1ec16 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -236,6 +236,9 @@ struct kvm_arch {
 	/* PMCR_EL0.N value for the guest */
 	u8 pmcr_n;
 
+	/* Limit value of PMCR_EL0.N for the guest */
+	u8 pmcr_n_limit;
+
 	/* Hypercall features firmware registers' descriptor */
 	struct kvm_smccc_features smccc_feat;
 
diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index 3053c06db7a9..ff4ec678afbd 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -890,6 +890,7 @@ int kvm_arm_set_vm_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
 	 * while the latter does not.
 	 */
 	kvm->arch.pmcr_n = arm_pmu->num_events - 1;
+	kvm->arch.pmcr_n_limit = arm_pmu->num_events - 1;
 
 	return 0;
 }
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index aba93db29697..959bd142b797 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -949,6 +949,52 @@ 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)
+{
+	struct kvm *kvm = vcpu->kvm;
+	u64 new_n, mutable_mask;
+	int ret = 0;
+
+	new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val);
+
+	if (unlikely(new_n != kvm->arch.pmcr_n)) {
+		mutex_lock(&kvm->lock);
+		/*
+		 * The vCPU can't have more counters than the PMU
+		 * hardware implements.
+		 */
+		if (new_n <= kvm->arch.pmcr_n_limit)
+			kvm->arch.pmcr_n = new_n;
+		else
+			ret = -EINVAL;
+
+		mutex_unlock(&kvm->lock);
+		if (ret)
+			return ret;
+	}
+
+	/*
+	 * 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).
+	 */
+	mutable_mask = (ARMV8_PMU_PMCR_MASK | ARMV8_PMU_PMCR_N);
+	val &= mutable_mask;
+	val |= (__vcpu_sys_reg(vcpu, r->reg) & ~mutable_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)),					\
@@ -1723,7 +1769,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	{ SYS_DESC(SYS_SVCR), undef_access },
 
 	{ PMU_SYS_REG(SYS_PMCR_EL0), .access = access_pmcr, .reset = reset_pmcr,
-	  .reg = PMCR_EL0, .get_user = get_pmcr },
+	  .reg = PMCR_EL0, .get_user = get_pmcr, .set_user = set_pmcr },
 	{ PMU_SYS_REG(SYS_PMCNTENSET_EL0),
 	  .access = access_pmcnten, .reg = PMCNTENSET_EL0 },
 	{ PMU_SYS_REG(SYS_PMCNTENCLR_EL0),
-- 
2.39.1.581.gbfd45094c4-goog