On 3/3/2025 6:00 AM, Dongli Zhang wrote:
> 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.
>
> In additional, PMU MSRs are set only at levels >= KVM_PUT_RESET_STATE,
> excluding runtime. Therefore, updating these MSRs without stopping events
> should be acceptable.
Suppose this works for upcoming mediated vPMU as well? If so, please
mention it here. Thanks.
>
> Finally, 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.
>
> 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 <[email protected]>
> ---
> 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 c5911baef0..4902694129 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -4160,13 +4160,6 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
> }
>
> if (IS_INTEL_CPU(env) && has_pmu_version > 0) {
> - if (has_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]);
> @@ -4182,8 +4175,6 @@ 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,
