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

[Robin Murphy]

On 2021-12-17 18:14, Rob Herring wrote:
> On Fri, Dec 17, 2021 at 10:57 AM Robin Murphy <robin.murphy@xxxxxxx> 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.