Re: [RFC PATCH 02/41] perf: Support guest enter/exit interfaces

[Zhang, Xiong Y]

On 4/16/2024 8:48 PM, Liang, Kan wrote:
> 
> 
> On 2024-04-16 1:34 a.m., Zhang, Xiong Y wrote:
>>
>>
>> On 4/16/2024 12:03 AM, Liang, Kan wrote:
>>>
>>>
>>> On 2024-04-12 4:56 p.m., Liang, Kan wrote:
>>>>> What if perf had a global knob to enable/disable mediate PMU support?  Then when
>>>>> KVM is loaded with enable_mediated_true, call into perf to (a) check that there
>>>>> are no existing !exclude_guest events (this part could be optional), and (b) set
>>>>> the global knob to reject all new !exclude_guest events (for the core PMU?).
>>>>>
>>>>> Hmm, or probably better, do it at VM creation.  That has the advantage of playing
>>>>> nice with CONFIG_KVM=y (perf could reject the enabling without completely breaking
>>>>> KVM), and not causing problems if KVM is auto-probed but the user doesn't actually
>>>>> want to run VMs.
>>>> I think it should be doable, and may simplify the perf implementation.
>>>> (The check in the schedule stage should not be necessary anymore.)
>>>>
>>>> With this, something like NMI watchdog should fail the VM creation. The
>>>> user should either disable the NMI watchdog or use a replacement.
>>>>
>>>> Thanks,
>>>> Kan
>>>>> E.g. (very roughly)
>>>>>
>>>>> int x86_perf_get_mediated_pmu(void)
>>>>> {
>>>>> 	if (refcount_inc_not_zero(...))
>>>>> 		return 0;
>>>>>
>>>>> 	if (<system wide events>)
>>>>> 		return -EBUSY;
>>>>>
>>>>> 	<slow path with locking>
>>>>> }
>>>>>
>>>>> void x86_perf_put_mediated_pmu(void)
>>>>> {
>>>>> 	if (!refcount_dec_and_test(...))
>>>>> 		return;
>>>>>
>>>>> 	<slow path with locking>
>>>>> }
>>>
>>>
>>> I think the locking should include the refcount check and system wide
>>> event check as well.
>>> It should be possible that two VMs are created very close.
>>> The second creation may mistakenly return 0 if there is no lock.
>>>
>>> I plan to do something as below (not test yet).
>>>
>>> +/*
>>> + * Currently invoked at VM creation to
>>> + * - Check whether there are existing !exclude_guest system wide events
>>> + *   of PMU with PERF_PMU_CAP_MEDIATED_VPMU
>>> + * - Set nr_mediated_pmu to prevent !exclude_guest event creation on
>>> + *   PMUs with PERF_PMU_CAP_MEDIATED_VPMU
>>> + *
>>> + * No impact for the PMU without PERF_PMU_CAP_MEDIATED_VPMU. The perf
>>> + * still owns all the PMU resources.
>>> + */
>>> +int x86_perf_get_mediated_pmu(void)
>>> +{
>>> +	int ret = 0;
>>> +	mutex_lock(&perf_mediated_pmu_mutex);
>>> +	if (refcount_inc_not_zero(&nr_mediated_pmu_vms))
>>> +		goto end;
>>> +
>>> +	if (atomic_read(&nr_include_guest_events)) {
>>> +		ret = -EBUSY;
>>> +		goto end;
>>> +	}
>>> +	refcount_inc(&nr_mediated_pmu_vms);
>>> +end:
>>> +	mutex_unlock(&perf_mediated_pmu_mutex);
>>> +	return ret;
>>> +}
>>> +EXPORT_SYMBOL_GPL(x86_perf_get_mediated_pmu);
>>> +
>>> +void x86_perf_put_mediated_pmu(void)
>>> +{
>>> +	mutex_lock(&perf_mediated_pmu_mutex);
>>> +	refcount_dec(&nr_mediated_pmu_vms);
>>> +	mutex_unlock(&perf_mediated_pmu_mutex);
>>> +}
>>> +EXPORT_SYMBOL_GPL(x86_perf_put_mediated_pmu);
>>>
>>>
>>> Thanks,
>>> Kan
>> x86_perf_get_mediated_pmu() is called at vm_create(), x86_perf_put_mediated_pmu() is called at vm_destroy(), then system wide perf events without exclude_guest=1 can not be created during the whole vm life cycle (where nr_mediated_pmu_vms > 0 always), do I understand and use the interface correctly ?
> 
> Right, but it only impacts the events of PMU with the
> PERF_PMU_CAP_MEDIATED_VPMU.
> For other PMUs, the event with exclude_guest=1 can still be created.
> KVM should not touch the counters of the PMU without
> PERF_PMU_CAP_MEDIATED_VPMU.
> 
> BTW: I will also remove the prefix x86, since the functions are in the
> generic code.
> 
> Thanks,
> Kan
After userspace VMM call VCPU SET_CPUID() ioctl, KVM knows whether vPMU is enabled or not. If perf_get_mediated_pmu() is called at vm create, it is too early. 
it is better to let perf_get_mediated_pmu() track per cpu PMU state, so perf_get_mediated_pmu() can be called by kvm after vcpu_cpuid_set(). Note user space vmm may call SET_CPUID() on one vcpu multi times, then here refcount maybe isn't suitable. what's a better solution ?

thanks