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.zh...@oracle.com>
> Reviewed-by: Zhao Liu <zhao1....@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...@linux.intel.com>
