Re: [PATCH v6 9/9] target/i386/kvm: don't stop Intel PMU counters

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 6/24/2025 3:43 PM, Dongli Zhang wrote:
> 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 4bbdf996ef..207de21404 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -4186,13 +4186,6 @@ 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]);
> @@ -4208,8 +4201,6 @@ 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,

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>