On 3/6/2025 3:00 AM, dongli.zhang@xxxxxxxxxx wrote:
> Hi Dapeng,
>
> On 3/4/25 11:35 PM, Mi, Dapeng wrote:
>> 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.
> TBH I am not sure if it works for mediated vPMU. The entire patchset is
> based the current implementation in mainline linux kernel.
>
> Otherwise, it is also required to modify the AMD's implementation ... that
> is, to stop AMD general PMCs or global registers (PerfMonV2).
>
> How about only consider the case without mediated vPMU so far?
>
> 1. For user without PerfMonV2 servers, they only need the patchset to reset
> general PMCs.
>
> 2. For user with PerfMonV2 servers, they need extra patch to reset global
> registers.
>
> 3. For mediated vPMU, we may add extra patch in the future.
I suppose it should be fine for mediated vPMU but I have no bandwidth to
thoroughly test it.
Ok, I think it's fine not to consider mediated vPMU in this patch series,
we can look at it later. Thanks.
>
> Thank you very much!
>
> Dongli Zhang
>
>>
>>> 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 <dongli.zhang@xxxxxxxxxx>
>>> ---
>>> 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,
