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

[dongli.zhang@oracle.com]

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.

> 
> 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;
> +            }
> +        }
> +    }
> +
>      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