Re: [PATCH v2 4/6] Documentation: arm64: document support for the AMU extension

[Valentin Schneider <valentin.schneider@arm.com>]

On 28/01/2020 16:53, Ionela Voinescu wrote:
> Or you won't be able to access them at all. Lacking firmware support
> accesses to AMU registers could be trapped in EL3. If access for EL1 and
> EL2 is enabled from EL3, it's still possible that the counters
> themselves are not enabled - that means they are not enabled to count
> the events they are designed to be counting. That's why in this case the
> event counter register could read 0.
> 
> But if we read 0, it does not necessarily mean that the counter is
> disabled. It could also mean that the events is meant to count did not
> happen yet.
> 

Right, which (as we discussed offline) is quite likely to happen if/when
we get stuff like SVE counters and we try to read them at boot time. Might
be worth adding a small note about that (0 != disabled).

>> I haven't seen something that would try to catch this on the kernel side.
>> Can we try to detect that (e.g. at least one counter returns > 0) in
>> cpu_amu_enable() and thus not write to the CPU-local 'amu_feat'?
>>
> 
> I'm reluctant to do this especially given that platforms might choose to
> keep some counters disabled while enabling some counters that might not
> have counted any events by the time we reach cpu_enable. We would end up
> mistakenly disabling the feature. I would rather leave the validation of
> the counters to be done at the location and for the purpose of their
> use: see patch 6/6 - the use of counters for frequency invariance.
> 

Hmph, I'm a bit torn on that one. It would be really nice to provide *some*
amount of sanity checking at core level - e.g. by checking that at least
one of the four architected counters reads non-zero. But as you say these
could be disabled, while some other arch/aux counter is enabled, and we
could then mistakenly disable the feature. So we can't really do much
unless we handle *each* individual counter. Oh well :/

>> While we're on the topic of detecting broken stuff, what if some CPUs
>> implement some auxiliary counters that some others don't?
>>
> 
> I think it should be up to the user of that counter to decide if the
> usecase is at CPU level or system level. My intention of this base
> support was to keep it simple and allow users of some counters to
> decide on their own how to validate and make use of either architected
> or auxiliary counters.
> 
> For example, in the case of frequency invariance, given a platform that
> does not support cpufreq based invariance, I would validate all CPUs for
> the use of AMU core and constant counters. If it happens that some CPUs
> do not support those counters or they are not enabled, we'd have to
> disable frequency invariance at system level.
> 
> For some other scenarios only partial support is needed - only a subset
> of CPUs need to support the counters for their use to be feasible.
> 
> But I believe only the user of the counters can decide, whether this is
> happening in architecture code, driver code, generic code.
> 

Right, the FIE support is actually a good example of that, I think.