Re: Heads up: official removal of older AMD (pre-Zen1) support from libpmc/hwpmc

On 4/21/26 13:49, Lionel Cons wrote:
> On Tue, 21 Apr 2026 at 18:24, Adrian Chadd <[email protected]> wrote:
>>
>> On Tue, 21 Apr 2026 at 08:41, Mitchell Horne <[email protected]> wrote:
>>>
>>> Hello,
>>>
>>> As the title suggests, I have proposed a change [1] to officially remove
>>> the older AMD processor event definitions and logic from the library and
>>> module. This means that processors older than the Zen 1 generation
>>> (~2017) will not be supported by these tools.
>>>
>>> I say "officially" because the functionality has been silently disabled
>>> for these older processors since 2020 at least [2].
>>>
>>> Among the many known issues, bugs, and sharp-edges relating to hwpmc(4)
>>> and its tools, I am not aware of this complaint being raised or reported
>>> anywhere. (Not to say that users haven't experienced this frustrating
>>> silent failure.)
>>>
>>
>> I mostly object, mostly because I'd rather we figure out why we've regressed in
>> this area and try to fix it to stop it happening in the future.
>>
>> I have a variety of AMD hardware sprinkled around at home going back to K7
>> era stuff that we still boot/run on (and heck I remember debugging our
>> earlier AMD counter programming bugs way back when.)
>>
>> Please don't take "noone has complained" as "noone is using this so we can
>> delete it" - I would take it as "everything is utterly broken in stupid ways and
>> we should fix the utterly broken stuff and figure out how to make sure it
>> isn't broken over time in the future."
> 
> I also object, mostly because I stumbled across the issue that my
> organisation has several large clusters with older AMD CPUs with
> FreeBSD in use [*]
> 
> [*] How, why, who, still under investigation since we found out. It's
> in legitimate use, running recent FreeBSD, and couple of department
> heads wanted to hang me when we touched it. So yes, old stuff is being
> used, and no money to replace it for now.
> 
> Lionel
> 

Cool, understood. I guess your insight is still incomplete into this,
but can you expand briefly on the use-case of these machines?

Is it likely that they require the use of hardware performance
monitoring, i.e. pmcstat(8)? As I've noted, it would be non-functional
in CURRENT without local modifications.

Best,
Mitchell