Re: [External] Re: [RFC PATCH 0/7] riscv: iommu: Add QoS ID support for resctrl device assignment

Hi Drew, Ben, and Qinxin,

On 8/6/26 4:42 PM, Ben Horgan wrote:
> Hi Drew,
>
> On 06/08/2026 06:23, Drew Fustini wrote:
>> On Tue, Jul 14, 2026 at 09:06:50PM +0800, Zhanpeng Zhang wrote:
>>> This RFC extends RISC-V CBQRI/resctrl support to external requesters
>>> behind a RISC-V IOMMU.
>> [..]
>>> Open questions for this RFC:
>>>
>>>    * whether a common resctrl "devices" file is the right ABI for
>>>      associating external requesters with a resource group;
>>
>> I think a separate file is the right approach to avoid confusing iommu
>> groups with pids.
>>
>> I should note that James Morse had an ARM MPAM "extras" tree [1] which
>> carried an iommu_group assignment interface, using the same
>> iommu_group:<id> format, but writing it to the tasks file. However, Ben
>> Horgan clarified back in March [2] in a reply to Qinxin Xia that it is
>> no longer maintained. Ben also wrote that the interface would likely
>> change upstream. I've added Ben Horgan and Qinxin Xia to the CC list so
>> hopefully they can chime in.
>
> I haven't thought about the interface much but Reinette let me know here [1], with good reasons,
> that we should consider the format of the tasks file fixed.
>
> Thanks,
>
> Ben
>
> [1] https://lore.kernel.org/all/[email protected]/
>

Thanks for the useful links and context. I referred to the earlier Arm
MPAM IOMMU group assignment work in my test version as well.

Given that the tasks ABI should remain PID-only, a separate file for
IOMMU group assignment seems to be the reasonable direction.

>>
>> I plan to review the patches in this series further but I didn't want to
>> delay any further in pointing out the above.
>>
>> Thanks,
>> Drew
>>
>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/log/?h=mpam/snapshot%2Bextras/v6.18-rc1
>> [2] https://lore.kernel.org/lkml/[email protected]/
>

Thanks,
Zhanpeng