[PATCH v4 11/11] target/i386/kvm: don't stop Intel PMU counters

[Dongli Zhang <dongli.zhang@oracle.com>]

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>
---
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.

 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 13ad7de690..6396f65558 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -4172,13 +4172,6 @@ static int kvm_put_msrs(X86CPU *cpu, int 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]);
@@ -4194,8 +4187,6 @@ static int kvm_put_msrs(X86CPU *cpu, int 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