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

[Drew Fustini <fustini@kernel.org>]

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.
> 
> The RISC-V IOMMU QoS ID extension associates a resource-control ID
> (RCID) and monitoring ID (MCID) with IOMMU-related memory requests [2].
> For a translated device request, the IOMMU obtains the IDs from the
> device context translation attributes (DC.ta) and supplies them to the
> I/O bridge after a successful translation. The bridge and downstream
> CBQRI-controlled resources can then apply their existing allocation and
> monitoring policy to the request.
> 
> The series is based directly on Drew Fustini's v4 RISC-V Ssqosid and
> CBQRI resctrl series [1]. That dependency provides CPU-side QoS ID
> plumbing and CBQRI-backed resctrl support. This series extends the same
> resctrl identity to device requests translated by a RISC-V IOMMU.
> 
> The RISC-V names map to the existing resctrl identifiers as follows:
> 
>     resctrl CLOSID  ->  RISC-V RCID
>     resctrl RMID    ->  RISC-V MCID
> 
> Userspace first creates a resctrl resource group. The new "devices"
> file then assigns an external requester to that group:
> 
>     userspace
>        |
>        | mkdir /sys/fs/resctrl/<group>
>        |
>        | echo "iommu_group:<id>" > /sys/fs/resctrl/<group>/devices
>        v
>     generic resctrl devices file
>        |
>        | (closid, rmid)
>        v
>     RISC-V CBQRI IOMMU binding backend
>        |
>        | (rcid, mcid)
>        v
>     RISC-V IOMMU DC.ta.{RCID,MCID}
> 
> The "devices" file is an assignment and membership interface only. It
> does not describe a new resctrl resource, schema, or monitoring domain;
> resource policy remains in the existing schemata and info files.
> 
> Common IOMMU helpers:
> 
> The userspace ABI names an IOMMU group by the numeric ID already used by
> /sys/kernel/iommu_groups. Patch 1 adds iommu_group_get_by_id() as a
> narrow, reference-counted lookup for that existing identifier.
> 
> Updating a group also needs stronger iteration semantics than separate
> iommu_group_for_each_dev() calls can provide. All devices must be
> validated before any device context is changed, and group membership
> must not change between validation and update. Patch 2 therefore adds
> iommu_group_update_devices():
> 
>     lock group->mutex
>       for each device: check()          /* may fail, no state changed */
>       for each device: update()         /* non-failing callback */
>     unlock group->mutex
> 
> The helper contains no resctrl or QoS policy. It only provides an
> all-or-none validation boundary for a group-wide update while keeping
> IOMMU group internals private. The RISC-V IOMMU QoS code is its first
> caller.
> 
> RISC-V IOMMU programming:
> 
> For translated device requests, the per-device-context QoS IDs are
> programmed in DC.ta. Active contexts are invalidated after an update so
> cached translations do not retain stale IDs. BARE mode cannot safely
> represent per-IOMMU-group assignment because only the per-IOMMU
> iommu_qosid global default is available, so group assignment rejects
> BARE mode.
> 
> The RISC-V IOMMU also has a per-IOMMU iommu_qosid register. The series
> exposes it as:
> 
>     /sys/class/iommu/<iommu>/qosid
> 
> This sysfs attribute programs the global RCID and MCID used for
> IOMMU-originated DDT, CQ, FQ, PQ, and MSI accesses. In BARE mode,
> device-originated requests also use these IDs. It is not a resctrl group
> assignment interface.
> 
> Open questions for this RFC:
> 
>   * whether a common resctrl "devices" file is the right ABI for
>     associating external requesters with a resource group;
>   * whether iommu_group_get_by_id() is an acceptable narrow common
>     helper for resolving the numeric IOMMU group ID passed through that
>     ABI; and
>   * whether iommu_group_update_devices() provides the right common
>     contract for checked group-wide updates.
> 
> [1] Drew Fustini's CBQRI v4 series:
> https://lore.kernel.org/linux-riscv/20260706-dfustini-atl-sc-cbqri-dt-v4-0-e75c20201d64@kernel.org/

FYI - the 3 Ssqosid patches from my v6 series have been merged into
mainline:

3ec6f50a91bd ("dt-bindings: riscv: Add Ssqosid extension description")
5a17eb5f1c4c ("riscv: Detect the Ssqosid extension")
326ba66db949 ("riscv: Add support for srmcfg CSR from Ssqosid extension")

I'll send a rebased series once v7.3-rc1 is tagged.

Thanks,
Drew