Re: [PATCH] ACPI/PPTT: Handle architecturally unknown cache types

[Sudeep Holla <sudeep.holla@arm.com>]

On 12/09/18 15:48, Jeffrey Hugo wrote:
> On 9/12/2018 4:49 AM, Sudeep Holla wrote:
>>
>>
>> On 11/09/18 21:38, Jeffrey Hugo wrote:
>>> On 9/11/2018 2:16 PM, Jeremy Linton wrote:
>>>> Hi Jeffrey,
>>>>
>>>> (+Sudeep)
>>>>
>>
>> [..]
>>
>>>>
>>>> If you look at the next line of code following this comment its going
>>>> to update the cache type for fully populated PPTT nodes. Although with
>>>> the suggested change its only going to activate if someone completely
>>>> fills out the node and fails to set the valid flag on the cache type.
>>>
>>> Yes, however that case doesn't apply to the scenario we are concerned
>>> about, doesn't seem to be fully following the PPTT spec, and seems odd
>>> that Linux just assumes that a "fully specified" cache is unified.
>>>
>>
>> Agreed, but adding code that will never get used is also not so good.
>> Do you have systems where this is needed ?
> 
> Yes.
> 

Not exactly IMO. I was referring to architecturally not specified
separate data/inst cache.

>>
>>>> What I suspect is happening in the reported case is that the nodes in
>>>> the PPTT table are missing fields we consider to be important. Since
>>>> that data isn't being filled out anywhere else, so we leave the cache
>>>> type alone too. This has the effect of hiding sysfs nodes with
>>>> incomplete information.
>>>>
>>>> Also, the lack of the DATA/INST fields is based on the assumption that
>>>> the only nodes which need their type field updated are outside of the
>>>> CPU core itself so they are pretty much guaranteed to be UNIFIED. Are
>>>> you hitting this case?
>>>>
>>>
>>> Yes.  Without this change, we hit the lscpu error in the commit message,
>>> and get zero output about the system.  We don't even get information
>>> about the caches which are architecturally specified or how many cpus
>>> are present.  With this change, we get what we expect out of lscpu (and
>>> also lstopo) including the cache(s) which are not architecturally
>>> specified.
>>>
>>
>> lscpu and lstopo are so broken. They just assume everything on CPU0.
>> If you hotplug them out, you start seeing issues. So reading and file
>> that doesn't exist and then bail out on other essential info though they
>> are present, hmmm ...
> 
> lscpu/lstopo being broken seems orthogonal to me.

Agreed.

> The kernel is not providing the type file because the kernel thinks the
> type is NOCACHE, despite PPTT providing a type.  In an ideal world, I
> think the kernel should be fixed (this change), and any deficiencies or
> bad assumptions in lscpu/lstopo should also be fixed.
> 

Again agreed as mentioned in the other mail. I just didn't like the
attribution of that issue to this kernel bug.

>>
>>> I guess I still don't understand why its important for PPTT to list, for
>>> example, the sets/ways of a cache in all scenarios.  In the case of a
>>> "transparent" cache (implementation defined as not reported per section
>>> D3.4.2 of the ARM ARM where the cache cannot be managed by SW), there
>>> may not be valid values for sets/ways.  I would argue its better to not
>>> report that information, rather than provide bogus information just to
>>> make Linux happy, which may break other OSes and provide means for which
>>> a user to hang themselves.
>>>
>>
>> While I agree presenting wrong info is not good, but one of the reasons
>> the cache info is present is to provide those info for some applications
>> to do optimization based on that(I am told so and not sure if just type
>> and size will be good enough) and you seem to agree with that below.
> 
> Yes, but if its not entirely clear, on my system, we have the
> information but its not being presented.  This change fixes that.  I'm
> willing to discuss an alternative fix, but to ignore the issue is not
> viable in my opinion.
> 

Agreed.

> What do we need to move forward here?
> 

Will the simple change I posted address the issue ? I don't want to give
an illusion that separate data/inst cache is support for architecturally
not specified caches. If it needs to be supported, then it needs to be
fixed correctly everywhere not just here.

-- 
Regards,
Sudeep