[PATCH RFC v2 0/2] RISC-V: Detect Ssqosid extension and handle sqoscfg CSR

[Drew Fustini]

This RFC series adds initial support for the Ssqosid extension and the
sqoscfg CSR as specified in Chapter 2 of the RISC-V Capacity and
Bandwidth Controller QoS Register Interface (CBQRI) specification [1].

QoS (Quality of Service) 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 there is an existing user
interface in Linux: the resctrl virtual filesystem [2].

The sqoscfg CSR provides a mechanism by which a software workload (e.g.
a process or a set of processes) can be associated with a resource
control ID (RCID) and a monitoring counter ID (MCID) that accompanies
each request made by the hart to shared resources like cache. CBQRI
defines operations to configure resource usage limits, in the form of
capacity or bandwidth, for an RCID. CBQRI also defines operations to
configure counters to track the resource utilization of an MCID.

The CBQRI spec is still in draft state and is undergoing review [3]. It
is possible there will be changes to the Ssqosid extension and the CBQRI
spec. For example, the CSR address for sqoscfg is not yet finalized.

My goal for this RFC is to determine if the 2nd patch is an acceptable
approach to handling sqoscfg when switching tasks. This RFC was tested
against a QEMU branch that implements the Ssqosid extension [4]. A test
driver [5] was used to set sqoscfg for the current process. This allows
__switch_to_sqoscfg() to be tested without resctrl.

This series is based on riscv/for-next at:

 b09313dd2e72 ("RISC-V: hwprobe: Explicity check for -1 in vdso init")

Changes from 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 [6]

[1] https://github.com/riscv-non-isa/riscv-cmqri/blob/main/riscv-cbqri.pdf
[2] https://docs.kernel.org/x86/resctrl.html
[3] https://lists.riscv.org/g/tech-cbqri/message/38
[4] https://gitlab.baylibre.com/baylibre/qemu/-/tree/riscv-cbqri-rfc-v2
[5] https://gitlab.baylibre.com/baylibre/linux/-/tree/riscv-sqoscfg-rfc-v2
[6] https://lore.kernel.org/linux-riscv/20230414155843.12963-1-andy.chiu@sifive.com/

---
Drew Fustini (1):
      RISC-V: Add support for sqoscfg CSR

Kornel Dulęba (1):
      RISC-V: Detect the Ssqosid extension

 arch/riscv/Kconfig                 | 19 +++++++++++++++++
 arch/riscv/include/asm/csr.h       |  8 +++++++
 arch/riscv/include/asm/hwcap.h     |  2 ++
 arch/riscv/include/asm/processor.h |  3 +++
 arch/riscv/include/asm/qos.h       | 43 ++++++++++++++++++++++++++++++++++++++
 arch/riscv/include/asm/switch_to.h |  3 +++
 arch/riscv/kernel/Makefile         |  1 +
 arch/riscv/kernel/cpu.c            |  1 +
 arch/riscv/kernel/cpufeature.c     |  1 +
 arch/riscv/kernel/qos/Makefile     |  2 ++
 arch/riscv/kernel/qos/qos.c        |  5 +++++
 11 files changed, 88 insertions(+)
---
base-commit: b09313dd2e726fe5e1fa574cd73f5e326c6030a4
change-id: 20230430-riscv-cbqri-rfc-v2-b007fcd19549

Best regards,
-- 
Drew Fustini <dfustini at baylibre.com>