Re: [PATCH 0/2] RISC-V: Detect Ssqosid extension and handle srmcfg CSR

[Conor Dooley <conor@kernel.org>]

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On Wed, Sep 10, 2025 at 11:15:28PM -0700, Drew Fustini wrote:
> This series adds support for the RISC-V Quality-of-Service Identifiers
> (Ssqosid) extension [1] which adds the srmcfg register. This CSR 
> configures a hart with two identifiers: a Resource Control ID (RCID)
> and a Monitoring Counter ID (MCID). These identifiers accompany each
> request issued by the hart to shared resource controllers.
> 
> Background on RISC-V QoS:
> 
> The Ssqosid extension is used by the RISC-V Capacity and Bandwidth
> Controller QoS Register Interface (CBQRI) specification [2]. QoS in
> this context is concerned with shared resources on an SoC such as cache
> capacity and memory bandwidth. Intel and AMD already have QoS features
> on x86 and ARM has MPAM. There is an existing user interface in Linux:
> the resctrl virtual filesystem [3].
> 
> The srmcfg CSR provides a mechanism by which a software workload (e.g.
> a process or a set of processes) can be associated with an RCID and an
> MCID. CBQRI defines operations to configure resource usage limits, in
> the form of capacity or bandwidth. CBQRI also defines operations to
> configure counters to track the resource utilization.
> 
> Goal for this series:
> 
> These two patches are taken from the implementation of resctrl support
> for RISC-V CBQRI. Please refer to the proof-of-concept RFC [4] for
> details on the resctrl implementation. More recently, I have rebased
> the CBQRI support on mainline [5]. Big thanks to James Morse for the
> tireless work to extract resctrl from arch/x86 and make it available
> to all archs.
> 
> I think it makes sense to first focus on the detection of Ssqosid and
> handling of srmcfg when switching tasks. It has been tested against a
> QEMU branch that implements Ssqosid and CBQRI [6]. A test driver [7]
> was used to set srmcfg for the current process. This allows switch_to
> to be tested without resctrl.
> 
> Changes from RFC v2:
>  - Rename all instances of the sqoscfg CSR to srmcfg to match the
>    ratified Ssqosid spec
>  - RFC v2: https://lore.kernel.org/linux-riscv/20230430-riscv-cbqri-rfc-v2-v2-0-8e3725c4a473@baylibre.com/
> 
> Changes from RFC v1:
>  - change DEFINE_PER_CPU to DECLARE_PER_CPU for cpu_sqoscfg in qos.h to
>    prevent linking error about multiple definition. Move DEFINE_PER_CPU
>    for cpu_sqoscfg into qos.c
>  - renamed qos prefix in function names to sqoscfg to be less generic
>  - handle sqoscfg the same way has_vector and has_fpu are handled in the
>    vector patch series
>  - RFC v1: https://lore.kernel.org/linux-riscv/20230410043646.3138446-1-dfustini@baylibre.com/
> 
> [1] https://github.com/riscv/riscv-ssqosid/releases/tag/v1.0
> [2] https://github.com/riscv-non-isa/riscv-cbqri/releases/tag/v1.0
> [3] https://docs.kernel.org/filesystems/resctrl.html
> [4] https://lore.kernel.org/linux-riscv/20230419111111.477118-1-dfustini@baylibre.com/
> [5] https://github.com/tt-fustini/linux/tree/b4/cbqri-v6-17-rc5
> [6] https://github.com/tt-fustini/qemu/tree/riscv-cbqri-rqsc-pptt
> [7] https://github.com/tt-fustini/linux/tree/ssqosid-v6-17-rc5-debug
> 
> Signed-off-by: Drew Fustini <fustini@kernel.org>
> ---
> Drew Fustini (2):
>       RISC-V: Detect the Ssqosid extension
>       RISC-V: Add support for srmcfg CSR from Ssqosid ext
> 
>  MAINTAINERS                        |  6 ++++++
>  arch/riscv/Kconfig                 | 17 ++++++++++++++++
>  arch/riscv/include/asm/csr.h       |  8 ++++++++
>  arch/riscv/include/asm/hwcap.h     |  1 +
>  arch/riscv/include/asm/processor.h |  3 +++
>  arch/riscv/include/asm/qos.h       | 41 ++++++++++++++++++++++++++++++++++++++
>  arch/riscv/include/asm/switch_to.h |  3 +++
>  arch/riscv/kernel/cpufeature.c     |  1 +

Why is there no binding change here? Is it not possible to use the
extension on DT systems, or is this an oversight?

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