Re: [PATCH kvm/queue v2 2/3] perf: x86/core: Add interface to query perfmon_event_map[] directly

[Like Xu <like.xu.linux@gmail.com>]

On 2/2/2022 10:43 pm, Peter Zijlstra wrote:
> On Mon, Jan 17, 2022 at 04:53:06PM +0800, Like Xu wrote:
>> From: Like Xu <likexu@tencent.com>
>>
>> Currently, we have [intel|knc|p4|p6]_perfmon_event_map on the Intel
>> platforms and amd_[f17h]_perfmon_event_map on the AMD platforms.
>>
>> Early clumsy KVM code or other potential perf_event users may have
>> hard-coded these perfmon_maps (e.g., arch/x86/kvm/svm/pmu.c), so
>> it would not make sense to program a common hardware event based
>> on the generic "enum perf_hw_id" once the two tables do not match.
>>
>> Let's provide an interface for callers outside the perf subsystem to get
>> the counter config based on the perfmon_event_map currently in use,
>> and it also helps to save bytes.
>>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Signed-off-by: Like Xu <likexu@tencent.com>
>> ---
>>   arch/x86/events/core.c            | 9 +++++++++
>>   arch/x86/include/asm/perf_event.h | 2 ++
>>   2 files changed, 11 insertions(+)
>>
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index 38b2c779146f..751048f4cc97 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -693,6 +693,15 @@ void x86_pmu_disable_all(void)
>>   	}
>>   }
>>   
>> +u64 perf_get_hw_event_config(int perf_hw_id)
>> +{
>> +	if (perf_hw_id < x86_pmu.max_events)
>> +		return x86_pmu.event_map(perf_hw_id);
>> +
>> +	return 0;
>> +}
> 
> Where does perf_hw_id come from? Does this need to be
> array_index_nospec() ?

A valid incoming parameter will be a member of the generic "enum perf_hw_id" table.

If array_index_nospec() helps, how about:

+u64 perf_get_hw_event_config(int hw_event)
+{
+	int max = x86_pmu.max_events;
+
+	if (hw_event < max)
+		return x86_pmu.event_map(array_index_nospec(hw_event, max));
+
+	return 0;
+}

> 
>> +EXPORT_SYMBOL_GPL(perf_get_hw_event_config);
> 
> Urgh... hate on kvm being a module again. We really need something like
> EXPORT_SYMBOL_KVM() or something.

As opposed to maintaining the obsolete {intel|amd}_event_mapping[] in the out 
context of perf,
a more appropriate method is to set up the table in the KVM through the new perf 
interface.

Well, I probably need Paolo's clarity to trigger more changes, whether it's 
introducing EXPORT_SYMBOL_KVM or a built-in KVM as a necessary prerequisite for 
vPMU.

> 
> 
>>   struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
>>   {
>>   	return static_call(x86_pmu_guest_get_msrs)(nr);
>> diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
>> index 8fc1b5003713..d1e325517b74 100644
>> --- a/arch/x86/include/asm/perf_event.h
>> +++ b/arch/x86/include/asm/perf_event.h
>> @@ -492,9 +492,11 @@ static inline void perf_check_microcode(void) { }
>>   
>>   #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_INTEL)
>>   extern struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr);
>> +extern u64 perf_get_hw_event_config(int perf_hw_id);
>>   extern int x86_perf_get_lbr(struct x86_pmu_lbr *lbr);
>>   #else
>>   struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr);
>> +u64 perf_get_hw_event_config(int perf_hw_id);
> 
> I think Paolo already spotted this one.

Indeed, I will apply it.

> 
>>   static inline int x86_perf_get_lbr(struct x86_pmu_lbr *lbr)
>>   {
>>   	return -1;
>> -- 
>> 2.33.1
>>