[PATCH v9 03/16] KVM: arm64: PMU: Freeze counter count after first run

[Akihiko Odaki]

PMU configuration is VM-scoped, but SET_NR_COUNTERS can be issued
through any vCPU. Checking only whether that vCPU's PMU has been
initialized allows userspace to change the counter count through an idle
sibling after another vCPU has run.

Reject the attribute once any vCPU has run. This keeps the VM-wide
implemented counter mask and the PMCR_EL0.N value stable after guest
execution begins.

Fixes: b7628c797376 ("KVM: arm64: Allow userspace to limit the number of PMU 
counters for EL2 VMs")
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Akihiko Odaki <od...@rsg.ci.i.u-tokyo.ac.jp>
---
 arch/arm64/kvm/pmu-emul.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index b5df6843dbcd..b4b877f31097 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -1087,6 +1087,9 @@ static int kvm_arm_pmu_v3_set_nr_counters(struct kvm_vcpu 
*vcpu, unsigned int n)
 {
        struct kvm *kvm = vcpu->kvm;
 
+       if (kvm_vm_has_ran_once(kvm))
+               return -EBUSY;
+
        if (!kvm->arch.arm_pmu)
                return -EINVAL;
 

-- 
2.55.0