linux-kernel - Re: [PATCH 2/6] cacheinfo: Set cache 'id' based on DT data

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Date:   Fri, 17 Dec 2021 19:08:33 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Rob Herring <robh@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        James Morse <james.morse@....com>,
        Jeremy Linton <jeremy.linton@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>, devicetree@...r.kernel.org,
        "open list:ACPI FOR ARM64 (ACPI/arm64)" <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH 2/6] cacheinfo: Set cache 'id' based on DT data

On 2021-12-17 18:14, Rob Herring wrote:
> On Fri, Dec 17, 2021 at 10:57 AM Robin Murphy <robin.murphy@....com> wrote:
>>
>> Hi Rob,
>>
>> On 2021-12-16 23:31, Rob Herring wrote:
>>> Use the minimum CPU h/w id of the CPUs associated with the cache for the
>>> cache 'id'. This will provide a stable id value for a given system. As
>>> we need to check all possible CPUs, we can't use the shared_cpu_map
>>> which is just online CPUs. There's not a cache to CPUs mapping in DT, so
>>> we have to walk all CPU nodes and then walk cache levels.
>>
>> I believe another expected use of the cache ID exposed in sysfs is to
>> program steering tags for cache stashing (typically in VFIO-based
>> userspace drivers like DPDK so we can't realistically mediate it any
>> other way). There were plans afoot last year to ensure that ACPI PPTT
>> could provide the necessary ID values for arm64 systems which will
>> typically be fairly arbitrary (but unique) due to reflecting underlying
>> interconnect routing IDs. Assuming that there will eventually be some
>> interest in cache stashing on DT-based systems too, we probably want to
>> allow for an explicit ID property on DT cache nodes in a similar manner.
> 
> If you have a suggestion for ID values that correspond to the h/w,
> then we can add them. I'd like a bit more than just trusting that ID
> is something real.
> 
> While the ACPI folks may be willing to take an arbitrary index, it's
> something we (mostly) avoid for DT.

Not really. On the CHI side there are two fields - StashNID, which could 
be any node ID value depending on the interconnect layout, plus 
(optionally) StashLPID to address a specific cache within that node if 
it's something like a CPU cluster. However, how a PCIe TLP steering tag 
translates to those fields in the resulting CHI flit is largely up to 
the root complex.

I think it's going to be more like a "reg" property than a nice 
validatable index.

>> That said, I think it does make sense to have some kind of
>> auto-generated fallback scheme *as well*, since I'm sure there will be
>> plenty systems which care about MPAM but don't support stashing, and
>> therefore wouldn't have a meaningful set of IDs to populate their DT
>> with. Conversely I think that might also matter for ACPI too - one point
>> I remember from previous discussions is that PPTT may use a compact
>> representation where a single entry represents all equivalent caches at
>> that level, so I'm not sure we can necessarily rely on IDs out of that
>> path being unique either.
> 
> AIUI, cache ids break the compact representation.

Right, firmware authors can't use it if they do want to specify IDs, but 
that also means that if we find we *are* consuming a compact PPTT, then 
chances are we're not getting meaningful IDs out of it for MPAM to rely on.

Robin.