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

Fenghua Yu
2025-10-03T17:56:59Z

On 9/10/25 13:42, 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>

Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>

Thanks.

-Fenghua