[PATCH v2 3/8] KVM: arm64: PMU: Preserve vCPU's PMCR_EL0.N value on vCPU reset

[Reiji Watanabe]

The number of PMU event counters is indicated in PMCR_EL0.N.
For a vCPU with PMUv3 configured, its value will be the same as
the host value by default. Userspace can set PMCR_EL0.N for the
vCPU to a lower value than the host value using KVM_SET_ONE_REG.
However, it is practically unsupported, as reset_pmcr() resets
PMCR_EL0.N to the host value on vCPU reset.

Change reset_pmcr() to preserve the vCPU's PMCR_EL0.N value on
vCPU reset so that userspace can limit the number of the PMU
event counter on the vCPU.

Signed-off-by: Reiji Watanabe <reijiw at google.com>
---
 arch/arm64/kvm/pmu-emul.c | 6 ++++++
 arch/arm64/kvm/sys_regs.c | 4 +++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index 24908400e190..937a272b00a5 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -213,6 +213,12 @@ void kvm_pmu_vcpu_init(struct kvm_vcpu *vcpu)
 
 	for (i = 0; i < ARMV8_PMU_MAX_COUNTERS; i++)
 		pmu->pmc[i].idx = i;
+
+	/*
+	 * Initialize PMCR_EL0 for the vCPU with the host value so that
+	 * the value is available at the very first vCPU reset.
+	 */
+	__vcpu_sys_reg(vcpu, PMCR_EL0) = read_sysreg(pmcr_el0);
 }
 
 /**
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 4959658b502c..67c1bd39b478 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -637,8 +637,10 @@ static void reset_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
 	if (!kvm_arm_support_pmu_v3())
 		return;
 
+	/* PMCR_EL0 for the vCPU is set to the host value at vCPU creation. */
+
 	/* Only preserve PMCR_EL0.N, and reset the rest to 0 */
-	pmcr = read_sysreg(pmcr_el0) & (ARMV8_PMU_PMCR_N_MASK << ARMV8_PMU_PMCR_N_SHIFT);
+	pmcr = __vcpu_sys_reg(vcpu, r->reg) & (ARMV8_PMU_PMCR_N_MASK << ARMV8_PMU_PMCR_N_SHIFT);
 	if (!kvm_supports_32bit_el0())
 		pmcr |= ARMV8_PMU_PMCR_LC;
 
-- 
2.39.0.314.g84b9a713c41-goog