[PATCH v7 9/9] target/i386/kvm: don't stop Intel PMU counters

Dongli Zhang
2025-11-11T06:20:26Z

PMU MSRs are set by QEMU only at levels >= KVM_PUT_RESET_STATE,
excluding runtime. Therefore, updating these MSRs without stopping events
should be acceptable.

In addition, KVM creates kernel perf events with host mode excluded
(exclude_host = 1). While the events remain active, they don't increment
the counter during QEMU vCPU userspace mode.

Finally, The kvm_put_msrs() sets the MSRs using KVM_SET_MSRS. The x86 KVM
processes these MSRs one by one in a loop, only saving the config and
triggering the KVM_REQ_PMU request. This approach does not immediately stop
the event before updating PMC. This approach is true since Linux kernel
commit 68fb4757e867 ("KVM: x86/pmu: Defer reprogram_counter() to
kvm_pmu_handle_event"), that is, v6.2.

No Fixed tag is going to be added for the commit 0d89436786b0 ("kvm:
migrate vPMU state"), because this isn't a bugfix.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
---
Changed since v3:
- Re-order reasons in commit messages.
- Mention KVM's commit 68fb4757e867 (v6.2).
- Keep Zhao's review as there isn't code change.
Changed since v6:
- Add Reviewed-by from Dapeng Mi.

target/i386/kvm/kvm.c | 9 ---------
1 file changed, 9 deletions(-)

diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 5258023fe7..d0df53807f 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -4213,13 +4213,6 @@ static int kvm_put_msrs(X86CPU *cpu, KvmPutState level)
}

if ((IS_INTEL_CPU(env) || IS_ZHAOXIN_CPU(env)) && pmu_version > 0) {
- if (pmu_version > 1) {
- /* Stop the counter. */
- kvm_msr_entry_add(cpu, MSR_CORE_PERF_FIXED_CTR_CTRL, 0);
- kvm_msr_entry_add(cpu, MSR_CORE_PERF_GLOBAL_CTRL, 0);
- }
-
- /* Set the counter values. */
for (i = 0; i < num_pmu_fixed_counters; i++) {
kvm_msr_entry_add(cpu, MSR_CORE_PERF_FIXED_CTR0 + i,
env->msr_fixed_counters[i]);
@@ -4235,8 +4228,6 @@ static int kvm_put_msrs(X86CPU *cpu, KvmPutState level)
env->msr_global_status);
kvm_msr_entry_add(cpu, MSR_CORE_PERF_GLOBAL_OVF_CTRL,
env->msr_global_ovf_ctrl);
-
- /* Now start the PMU. */
kvm_msr_entry_add(cpu, MSR_CORE_PERF_FIXED_CTR_CTRL,
env->msr_fixed_ctr_ctrl);
kvm_msr_entry_add(cpu, MSR_CORE_PERF_GLOBAL_CTRL,
--
2.39.3