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Message-ID: <998fd76f-2bd9-4492-bf2e-e8cd981df67f@linux.intel.com>
Date: Tue, 16 Apr 2024 13:34:20 +0800
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Subject: Re: [RFC PATCH 02/41] perf: Support guest enter/exit interfaces
To: "Liang, Kan" <kan.liang@linux.intel.com>,
 Sean Christopherson <seanjc@google.com>
Cc: pbonzini@redhat.com, peterz@infradead.org, mizhang@google.com,
 kan.liang@intel.com, zhenyuw@linux.intel.com, dapeng1.mi@linux.intel.com,
 jmattson@google.com, kvm@vger.kernel.org, linux-perf-users@vger.kernel.org,
 linux-kernel@vger.kernel.org, zhiyuan.lv@intel.com, eranian@google.com,
 irogers@google.com, samantha.alt@intel.com, like.xu.linux@gmail.com,
 chao.gao@intel.com
References: <20240126085444.324918-1-xiong.y.zhang@linux.intel.com>
 <20240126085444.324918-3-xiong.y.zhang@linux.intel.com>
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 <23af8648-ca9f-41d2-8782-f2ffc3c11e9e@linux.intel.com>
 <ZhmIrQQVgblrhCZs@google.com>
 <2342a4e2-2834-48e2-8403-f0050481e59e@linux.intel.com>
 <ab2953b7-18fd-4b4c-a83b-ab243e2a21e1@linux.intel.com>
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From: "Zhang, Xiong Y" <xiong.y.zhang@linux.intel.com>
In-Reply-To: <ab2953b7-18fd-4b4c-a83b-ab243e2a21e1@linux.intel.com>
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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 ?

thanks