KVM allows userspace to write any value to MDCR_EL2.HPMN. However,
kvm_arm_set_nr_counters() rewrites HPMN for every vCPU whenever
userspace changes the PMU or its counter count. This can discard a
value previously restored with KVM_SET_ONE_REG.
The architecture only defines HPMN's value on warm reset. Stop
rewriting it after vCPU initialization and update nr_pmu_counters
directly instead. reset_mdcr() continues to initialize HPMN from the
counter count current at KVM_ARM_VCPU_INIT.
Fixes: c8823e51b534 ("KVM: arm64: Fix MDCR_EL2.HPMN reset value")
Closes:
https://sashiko.dev/#/patchset/20260706-hybrid-v8-0-de459617b59d%40rsg.ci.i.u-tokyo.ac.jp?part=6
Assisted-by: Codex:gpt-5.5
Signed-off-by: Akihiko Odaki <[email protected]>
---
arch/arm64/kvm/pmu-emul.c | 22 ++--------------------
1 file changed, 2 insertions(+), 20 deletions(-)
diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index 98305bbfc095..b5df6843dbcd 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -1022,30 +1022,12 @@ u8 kvm_arm_pmu_get_max_counters(struct kvm *kvm)
return bitmap_weight(arm_pmu->cntr_mask,
ARMV8_PMU_MAX_GENERAL_COUNTERS);
}
-static void kvm_arm_set_nr_counters(struct kvm *kvm, unsigned int nr)
-{
- kvm->arch.nr_pmu_counters = nr;
-
- /* Reset MDCR_EL2.HPMN behind the vcpus' back... */
- if (test_bit(KVM_ARM_VCPU_HAS_EL2, kvm->arch.vcpu_features)) {
- struct kvm_vcpu *vcpu;
- unsigned long i;
-
- kvm_for_each_vcpu(i, vcpu, kvm) {
- u64 val = __vcpu_sys_reg(vcpu, MDCR_EL2);
- val &= ~MDCR_EL2_HPMN;
- val |= FIELD_PREP(MDCR_EL2_HPMN,
kvm->arch.nr_pmu_counters);
- __vcpu_assign_sys_reg(vcpu, MDCR_EL2, val);
- }
- }
-}
-
static void kvm_arm_set_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu)
{
lockdep_assert_held(&kvm->arch.config_lock);
kvm->arch.arm_pmu = arm_pmu;
- kvm_arm_set_nr_counters(kvm, kvm_arm_pmu_get_max_counters(kvm));
+ kvm->arch.nr_pmu_counters = kvm_arm_pmu_get_max_counters(kvm);
}
/**
@@ -1111,7 +1093,7 @@ static int kvm_arm_pmu_v3_set_nr_counters(struct kvm_vcpu
*vcpu, unsigned int n)
if (n > kvm_arm_pmu_get_max_counters(kvm))
return -EINVAL;
- kvm_arm_set_nr_counters(kvm, n);
+ kvm->arch.nr_pmu_counters = n;
return 0;
}
--
2.55.0
