Re: [PATCH v7 00/11] arm_mpam: Add MPAM-Fb firmware support

Hi Andre,

On 8/4/26 6:50 PM, Andre Przywara wrote:
> Hi Gavin,
> 
> many thanks for the review and the testing!
> 
> ...
> 
> On 8/4/26 05:29, Gavin Shan wrote:
>> On 8/1/26 3:03 AM, Andre Przywara wrote:
>>> v7 is another quick respin of the MPAM-Fb code, for firmware based MSC
>>> accesses. This propagates MSC access errors in a function where it was
>>> missing before, covering more functions that pass their return value
>>> to userspace (new patch 07/11). Also some bug fixes, formatting
>>> adjustments and adding the accumulated tags - many thanks to the
>>> reviewers. Find the changelog below. Based on v7.2-rc1.
>>>
>>> =======================
>>> The Arm MPAM specification defines Memory System Components (MSCs),
>>> which are devices that are programmed through an MMIO register frame. In
>>> some occasions this turned out to be too limiting: the MSC might be
>>> located behind a separate bus system (for instance inside an on-board
>>> controller), it might be mapped secure-only, or in a different processor
>>> socket without direct MMIO mapping. Also the MMIO access might be too slow
>>> or it would need to be filtered or otherwise access controlled. Finally
>>> there might be bugs in the MSC integration, which require a mediating
>>> firmware to be accessible.
>>>
>>> To accommodate all those different use cases, the MPAM-Fb specification
>>> [1] describes an alternative way to access MSCs. Accesses to an MSC
>>> would be wrapped in a message and communicated to the system using a
>>> shared-memory/mailbox system mostly mimicking the Arm SCMI spec.
>>> For ACPI systems, this would be abstracted through an ACPI PCC channel,
>>> which provides the shared-memory region and the mailbox trigger. We can
>>> lean on existing ACPI parsing code to register with these two
>>> subsystems, but cannot rely on the existing SCMI code in the kernel.
>>> This means we somewhat need to open code a very simplified SCMI handler,
>>> which just provides enough functionality for the very basic subset of
>>> SCMI that the MPAM-Fb spec requires.
>>>
>>> The first seven patches rework all MSC access wrappers to propagate error
>>> information. Pure MMIO based MSC accesses would never fail, but the
>>> MPAM-Fb access can go wrong in multiple ways. The patches have been split
>>> up purely for reviewing reasons, if the number is a problem, we could as
>>> well squash them. Please note that until the very last patch of this series
>>> any MSC accesses would always only return 0, it's only the final enablement
>>> of MPAM-Fb that could possibly introduce errors. Hence all former patches
>>> can add error handling gradually, those code paths wouldn't be triggered
>>> before patch 11/11.
>>> Patch 8/11 solves a nasty problem: At the moment we protect stateful MSC
>>> register accesses (mon_sel) through a spinlock. Unfortunately the mailbox
>>> subsystem and the slow nature of the communication through this channel
>>> forbid MPAM-Fb access in atomic context. So this patch keeps using a
>>> spinlock for MMIO based accesses, but reverts to a mutex otherwise.
>>> We just deny taking the lock for MPAM-Fb in atomic context, ideally we
>>> wouldn't need that (no need to IPI another core when the MSC access does
>>> not need to be local to one particular core), or we simply deny that part
>>> of the functionality (access through perf).
>>> Patch 9/11 adds the code to redirect MSC accesses through the
>>> PCC shmem/mailbox system.
>>> Patch 10/11 reworks the error interrupt handler to use a threaded IRQ for
>>> MPAM-Fb, to be able to do MPAM-Fb MSC accesses inside (which might sleep).
>>> The final patch 11/11 then adds the code to detect and store the PCC
>>> channel information from the ACPI tables, and eventually enables
>>> MPAM-Fb accesses.
>>>
>>> This would enable systems where some MSCs are not accessible via MMIO to
>>> use those components anyway.
>>>
>>> Please have a look and test!
>>>
>>> Cheers,
>>> Andre
>>>
>>> [1] https://developer.arm.com/documentation/den0144/latest
>>>
>>> Changes in v7:
>>> - add tags
>>> - add new patch to propagate errors in mpam_reprogram_ris_partid()
>>> - prevent loop when mpam_diable() tries MSC accesses again
>>> - register MMIO error IRQ handler without IRQF_ONESHOT
>>> - re-use existing "err" variable instead of declaring "ret"
>>> - initialise mon_sel_lock later, to wait for interface decision
>>> - convert timeout units for nominal latency, between us and ms
>>
>> [...]
>>
>>>
>>> Andre Przywara (11):
>>>    arm_mpam: let low level MSC accessors return an error
>>>    arm_mpam: propagate MSC access errors for hw_probe functions
>>>    arm_mpam: propagate MSC access errors for MBWU counters
>>>    arm_mpam: propagate MSC access errors for msmon helpers
>>>    arm_mpam: propagate MSC access errors for __ris_msmon_read()
>>>    arm_mpam: propagate MSC access errors for state saving function
>>>    arm_mpam: propagate MSC access errors for mpam_reprogram_ris_partid()
>>>    arm_mpam: prepare mon_sel locking for MPAM-Fb
>>>    arm_mpam: add MPAM-Fb MSC firmware access support
>>>    arm_mpam: change MPAM-Fb error IRQ to use a threaded IRQ handler
>>>    arm_mpam: detect and enable MPAM-Fb PCC support
>>>
>>>   drivers/resctrl/Makefile        |   2 +-
>>>   drivers/resctrl/mpam_devices.c  | 820 +++++++++++++++++++++++---------
>>>   drivers/resctrl/mpam_fb.c       | 250 ++++++++++
>>>   drivers/resctrl/mpam_internal.h |  62 ++-
>>>   include/linux/arm_mpam.h        |   2 +-
>>>   5 files changed, 915 insertions(+), 221 deletions(-)
>>>   create mode 100644 drivers/resctrl/mpam_fb.c
>>>
>>
>> With this series (manually) applied to v7.2.rc6, the tests for the existing functions
>> like kunit-tests, L3 cache partitioning, MBW (soft) limiting, llc_occupancy monitor
>> look fine on NVidia's grace-hopper machine.
> 
> Awesome, many thanks for doing that!
> 

Glad to give it a try :-)

>> Tested-by: Gavin Shan <[email protected]>
> 
> I'd assume this is a regression test, as your machine uses the standard MMIO interface?
> If you don't mind, I'd put this tag on the first refactoring patches, so 01/11 - 08/11, and leave out the remaining 3, as they are about MPAM-Fb, which wasn't really tested, I guess? Also patch 10/11 (the error IRQ changes) changed, so I would drop all tags from this anyway.
> 

Yes, it's a regression test. My Tested-by tag are only applied to 01/11 - 08/11.
I don't have a machine where MPAM-fb is available to test 09/11 - 11/11.

Thanks,
Gavin

> Cheers,
> Andre
>