Re: [PATCH v2 10/29] arm_mpam: Add cpuhp callbacks to probe MSC hardware

Ben Horgan
2025-09-12T10:42:11Z

Hi James,

On 9/10/25 21:42, James Morse wrote:
> Because an MSC can only by accessed from the CPUs in its cpu-affinity
> set we need to be running on one of those CPUs to probe the MSC
> hardware.
>
> Do this work in the cpuhp callback. Probing the hardware will only
> happen before MPAM is enabled, walk all the MSCs and probe those we can
> reach that haven't already been probed as each CPU's online call is made.
>
> This adds the low-level MSC register accessors.
>
> Once all MSCs reported by the firmware have been probed from a CPU in
> their respective cpu-affinity set, the probe-time cpuhp callbacks are
> replaced. The replacement callbacks will ultimately need to handle
> save/restore of the runtime MSC state across power transitions, but for
> now there is nothing to do in them: so do nothing.
>
> The architecture's context switch code will be enabled by a static-key,
> this can be set by mpam_enable(), but must be done from process context,
> not a cpuhp callback because both take the cpuhp lock.
> Whenever a new MSC has been probed, the mpam_enable() work is scheduled
> to test if all the MSCs have been probed. If probing fails, mpam_disable()
> is scheduled to unregister the cpuhp callbacks and free memory.
>
> CC: Lecopzer Chen <lecopzerc@nvidia.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> Changes since v1:
> * Removed register bounds check. If the firmware tables are wrong the
> resulting translation fault should be enough to debug this.
> * Removed '&' in front of a function pointer.
> * Pulled mpam_disable() into this patch.
> * Disable mpam when probing fails to avoid extra work on broken platforms.
> * Added mpam_disbale_reason as there are now two non-debug reasons for this
> to happen.

Looks good to me.

Reviewed-by: Ben Horgan <ben.horgan@arm.com>

Thanks,

Ben