Re: [RFC PATCH 00/36] arm_mpam: Add basic mpam driver

[James Morse]

Hi Jonathan,

On 01/08/2025 17:09, Jonathan Cameron wrote:
> On Fri, 11 Jul 2025 18:36:12 +0000
> James Morse <james.morse@xxxxxxx> wrote:
> > This is just enough MPAM driver for the ACPI and DT pre-requisites.
> > It doesn't contain any of the resctrl code, meaning you can't actually drive it
> > from user-space yet.

[..]

> Whilst I get that this is minimal, I was a bit surprised that it doesn't
> contain enough to have the driver actually bind to the platform devices
> I think that needs the CPU hotplug handler to register a requester.
> So about another 4 arch patches from your tree.  Maybe you can shuffle
> things around to help with that.

Ah, I hadn't spotted that. The register-requestor code should only serve to
reduce the available PARTID - just in case the CPUs support less than the
cache hierarchy.
It's likely its the 'system_supports_mpam()' that prevents the driver being
registered. I'll move that into the arm64 patches - its needed so any id
register overrides in the CPU knock out the driver too.


> That makes this a pain to test in isolation.
>
> Given desire to poke the corners, I'm rebasing the old QEMU emulation and
> will poke it some more.  Now we are getting close to upstream kernel support
> maybe I'll even clean that up for potential upstream QEMU.
>
> For bonus points I 'could' hook it up to the cache simulator and actually
> generate real 'counts' but that's probably more for fun than because it's
> useful. Fake numbers are a lot cheaper to get.

I've not found a good source of fake numbers to use. Ideally it would just
increment whenever the task was scheduled - but I've found that hard to hack
that up in linux.
I fall back to reading the counters instead of the MPAM registers ... but its
hard to test for overflow or double counting with that.


Thanks,

James