Re: [PATCH 2/7] target/i386/kvm: introduce 'pmu-cap-disabled' to set KVM_PMU_CAP_DISABLE

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

On 11/21/2024 6:06 PM, Mi, Dapeng wrote:
> On 11/8/2024 7:44 AM, dongli.zhang@oracle.com wrote:
>> Hi Zhao,
>>
>>
>> On 11/6/24 11:52 PM, Zhao Liu wrote:
>>> (+Dapang & Zide)
>>>
>>> Hi Dongli,
>>>
>>> On Mon, Nov 04, 2024 at 01:40:17AM -0800, Dongli Zhang wrote:
>>>> Date: Mon,  4 Nov 2024 01:40:17 -0800
>>>> From: Dongli Zhang <dongli.zhang@oracle.com>
>>>> Subject: [PATCH 2/7] target/i386/kvm: introduce 'pmu-cap-disabled' to set
>>>>  KVM_PMU_CAP_DISABLE
>>>> X-Mailer: git-send-email 2.43.5
>>>>
>>>> The AMD PMU virtualization is not disabled when configuring
>>>> "-cpu host,-pmu" in the QEMU command line on an AMD server. Neither
>>>> "-cpu host,-pmu" nor "-cpu EPYC" effectively disables AMD PMU
>>>> virtualization in such an environment.
>>>>
>>>> As a result, VM logs typically show:
>>>>
>>>> [    0.510611] Performance Events: Fam17h+ core perfctr, AMD PMU driver.
>>>>
>>>> whereas the expected logs should be:
>>>>
>>>> [    0.596381] Performance Events: PMU not available due to virtualization, using software events only.
>>>> [    0.600972] NMI watchdog: Perf NMI watchdog permanently disabled
>>>>
>>>> This discrepancy occurs because AMD PMU does not use CPUID to determine
>>>> whether PMU virtualization is supported.
>>> Intel platform doesn't have this issue since Linux kernel fails to check
>>> the CPU family & model when "-cpu *,-pmu" option clears PMU version.
>>>
>>> The difference between Intel and AMD platforms, however, is that it seems
>>> Intel hardly ever reaches the “...due virtualization” message, but
>>> instead reports an error because it recognizes a mismatched family/model.
>>>
>>> This may be a drawback of the PMU driver's print message, but the result
>>> is the same, it prevents the PMU driver from enabling.
>>>
>>> So, please mention that KVM_PMU_CAP_DISABLE doesn't change the PMU
>>> behavior on Intel platform because current "pmu" property works as
>>> expected.
>> Sure. I will mention this in v2.
>>
>>>> To address this, we introduce a new property, 'pmu-cap-disabled', for KVM
>>>> acceleration. This property sets KVM_PMU_CAP_DISABLE if
>>>> KVM_CAP_PMU_CAPABILITY is supported. Note that this feature currently
>>>> supports only x86 hosts, as KVM_CAP_PMU_CAPABILITY is used exclusively for
>>>> x86 systems.
>>>>
>>>> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
>>>> ---
>>>> Another previous solution to re-use '-cpu host,-pmu':
>>>> https://urldefense.com/v3/__https://lore.kernel.org/all/20221119122901.2469-1-dongli.zhang@oracle.com/__;!!ACWV5N9M2RV99hQ!Nm8Db-mwBoMIwKkRqzC9kgNi5uZ7SCIf43zUBn92Ar_NEbLXq-ZkrDDvpvDQ4cnS2i4VyKAp6CRVE12bRkMF$ 
>>> IMO, I prefer the previous version. This VM-level KVM property is
>>> difficult to integrate with the existing CPU properties. Pls refer later
>>> comments for reasons.
>>>
>>>>  accel/kvm/kvm-all.c        |  1 +
>>>>  include/sysemu/kvm_int.h   |  1 +
>>>>  qemu-options.hx            |  9 ++++++-
>>>>  target/i386/cpu.c          |  2 +-
>>>>  target/i386/kvm/kvm.c      | 52 ++++++++++++++++++++++++++++++++++++++
>>>>  target/i386/kvm/kvm_i386.h |  2 ++
>>>>  6 files changed, 65 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
>>>> index 801cff16a5..8b5ba45cf7 100644
>>>> --- a/accel/kvm/kvm-all.c
>>>> +++ b/accel/kvm/kvm-all.c
>>>> @@ -3933,6 +3933,7 @@ static void kvm_accel_instance_init(Object *obj)
>>>>      s->xen_evtchn_max_pirq = 256;
>>>>      s->device = NULL;
>>>>      s->msr_energy.enable = false;
>>>> +    s->pmu_cap_disabled = false;
>>>>  }
>>> The CPU property "pmu" also defaults to "false"...but:
>>>
>>>  * max CPU would override this and try to enable PMU by default in
>>>    max_x86_cpu_initfn().
>>>
>>>  * Other named CPU models keep the default setting to avoid affecting
>>>    the migration.
>>>
>>> The pmu_cap_disabled and “pmu” property look unbound and unassociated,
>>> so this can cause the conflict when they are not synchronized. For
>>> example,
>>>
>>> -cpu host -accel kvm,pmu-cap-disabled=on
>>>
>>> The above options will fail to launch a VM (on Intel platform).
>>>
>>> Ideally, the “pmu” property and pmu-cap-disabled should be bound to each
>>> other and be consistent. But it's not easy because:
>>>  - There is no proper way to have pmu_cap_disabled set different default
>>>    values (e.g., "false" for max CPU and "true" for named CPU models)
>>>    based on different CPU models.
>>>  - And, no proper place to check the consistency of pmu_cap_disabled and
>>>    enable_pmu.
>>>
>>> Therefore, I prefer your previous approach, to reuse current CPU "pmu"
>>> property.
>> Thank you very much for the suggestion and reasons.
>>
>> I am going to follow your suggestion to switch back to the previous solution in v2.
> +1.
>
>  I also prefer to leverage current exist "+/-pmu" option instead of adding
> a new option. More options, more complexity. When they are not
> inconsistent, which has higher priority? all these are issues.
>
> Although KVM_CAP_PMU_CAPABILITY is a VM-level PMU capability, but all CPUs
> in a same VM should always share same PMU configuration (Don't consider
> hybrid platforms which have many issues need to be handled specifically).
>
>
>>> Further, considering that this is currently the only case that needs to
>>> to set the VM level's capability in the CPU context, there is no need to
>>> introduce a new kvm interface (in your previous patch), which can instead
>>> be set in kvm_cpu_realizefn(), like:
>>>
>>>
>>> diff --git a/target/i386/kvm/kvm-cpu.c b/target/i386/kvm/kvm-cpu.c
>>> index 99d1941cf51c..05e9c9a1a0cf 100644
>>> --- a/target/i386/kvm/kvm-cpu.c
>>> +++ b/target/i386/kvm/kvm-cpu.c
>>> @@ -42,6 +42,8 @@ static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
>>>  {
>>>      X86CPU *cpu = X86_CPU(cs);
>>>      CPUX86State *env = &cpu->env;
>>> +    KVMState *s = kvm_state;
>>> +    static bool first = true;
>>>      bool ret;
>>>
>>>      /*
>>> @@ -63,6 +65,29 @@ static bool kvm_cpu_realizefn(CPUState *cs, Error **errp)
>>>       *   check/update ucode_rev, phys_bits, guest_phys_bits, mwait
>>>       *   cpu_common_realizefn() (via xcc->parent_realize)
>>>       */
>>> +
>>> +    if (first) {
>>> +        first = false;
>>> +
>>> +        /*
>>> +         * Since Linux v5.18, KVM provides a VM-level capability to easily
>>> +         * disable PMUs; however, QEMU has been providing PMU property per
>>> +         * CPU since v1.6. In order to accommodate both, have to configure
>>> +         * the VM-level capability here.
>>> +         */
>>> +        if (!cpu->enable_pmu &&
>>> +            kvm_check_extension(s, KVM_CAP_PMU_CAPABILITY)) {
>>> +            int r = kvm_vm_enable_cap(s, KVM_CAP_PMU_CAPABILITY, 0,
>>> +                                      KVM_PMU_CAP_DISABLE);
>>> +
>>> +            if (r < 0) {
>>> +                error_setg(errp, "kvm: Failed to disable pmu cap: %s",
>>> +                           strerror(-r));
>>> +                return false;
>>> +            }
>>> +        }
> It seems KVM_CAP_PMU_CAPABILITY is called to only disable PMU here. From
> point view of logic completeness,  KVM_CAP_PMU_CAPABILITY should be called
> to enabled PMU as well when user wants to enable PMU.
>
> I know currently we only need to disable PMU, but we may need to enable PMU
> via KVM_CAP_PMU_CAPABILITY soon.
>
> We are working on the new KVM mediated vPMU framework, Sean suggest to
> leverage KVM_CAP_PMU_CAPABILITY to enable mediated vPMU dynamically
> (https://lore.kernel.org/all/Zz4uhmuPcZl9vJVr@google.com/). So It would be
> better if the enable logic can be added here as well.
>
> Thanks.

Hi Dongli,

May I know if you have plan to continue to update this patch recently? As
previous comment said, our KVM mediated vPMU solution needs qemu to
explicitly call KVM_CAP_PMU_CAPABILITY to enable mediated vPMU as well. If
you have no plan to update this patch recently, would you mind me to pick
up this patch
(https://lore.kernel.org/all/20221119122901.2469-2-dongli.zhang@oracle.com/)
and post with other our mediated vPMU related patches to enable mediated vPMU?

Thanks!

Dapeng Mi

>
>
>>> +    }
>>> +
>>>      if (cpu->max_features) {
>>>          if (enable_cpu_pm) {
>>>              if (kvm_has_waitpkg()) {
>>> ---
>> Sure. I will limit the change within only x86 + KVM.
>>
>>> In addition, if PMU is disabled, why not mask the perf related bits in
>>> 8000_0001_ECX? :)
>>>
>> My fault. I have masked only 0x80000022, and I forgot 0x80000001 for AMD.
>>
>> Thank you very much for the reminder.
>>
>>
>> I will wait for a day or maybe the weekend. I am going to switch to the previous
>> solution in v2 if there isn't any further objection with a more valid reason.
>>
>> Thank you very much for the feedback!
>>
>> Dongli Zhang
>>
>>