[RFC PATCH 0/5] RISC-V: Add dynamic TSO support

[Christoph Muellner]

From: Christoph Müllner <christoph.muellner at vrull.eu>

The upcoming RISC-V Ssdtso specification introduces a bit in the senvcfg
CSR to switch the memory consistency model at run-time from RVWMO to TSO
(and back). The active consistency model can therefore be switched on a
per-hart base and managed by the kernel on a per-process/thread base.

This patch implements basic Ssdtso support and adds a prctl API on top
so that user-space processes can switch to a stronger memory consistency
model (than the kernel was written for) at run-time.

I am not sure if other architectures support switching the memory
consistency model at run-time, but designing the prctl API in an
arch-independent way allows reusing it in the future.

The patchset also comes with a short documentation of the prctl API.

This series is based on the second draft of the Ssdtso specification
which was published recently on an RVI list:
  https://lists.riscv.org/g/tech-arch-review/message/183
Note, that the Ssdtso specification is in development state
(i.e., not frozen or even ratified) which is also the reason
why I marked the series as RFC.

One aspect that is not covered in this patchset is virtualization.
It is planned to add virtualization support in a later version.
Hints/suggestions on how to implement this part are very much
appreciated.

Christoph Müllner (5):
  RISC-V: Add basic Ssdtso support
  RISC-V: Expose Ssdtso via hwprobe API
  uapi: prctl: Add new prctl call to set/get the memory consistency
    model
  RISC-V: Implement prctl call to set/get the memory consistency model
  RISC-V: selftests: Add DTSO tests

 Documentation/arch/riscv/hwprobe.rst          |  3 +
 .../mm/dynamic-memory-consistency-model.rst   | 76 ++++++++++++++++++
 arch/riscv/Kconfig                            | 10 +++
 arch/riscv/include/asm/csr.h                  |  1 +
 arch/riscv/include/asm/dtso.h                 | 74 ++++++++++++++++++
 arch/riscv/include/asm/hwcap.h                |  1 +
 arch/riscv/include/asm/processor.h            |  8 ++
 arch/riscv/include/asm/switch_to.h            |  3 +
 arch/riscv/include/uapi/asm/hwprobe.h         |  1 +
 arch/riscv/kernel/Makefile                    |  1 +
 arch/riscv/kernel/cpufeature.c                |  1 +
 arch/riscv/kernel/dtso.c                      | 33 ++++++++
 arch/riscv/kernel/process.c                   |  4 +
 arch/riscv/kernel/sys_riscv.c                 |  1 +
 include/uapi/linux/prctl.h                    |  5 ++
 kernel/sys.c                                  | 12 +++
 tools/testing/selftests/riscv/Makefile        |  2 +-
 tools/testing/selftests/riscv/dtso/.gitignore |  1 +
 tools/testing/selftests/riscv/dtso/Makefile   | 11 +++
 tools/testing/selftests/riscv/dtso/dtso.c     | 77 +++++++++++++++++++
 20 files changed, 324 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/mm/dynamic-memory-consistency-model.rst
 create mode 100644 arch/riscv/include/asm/dtso.h
 create mode 100644 arch/riscv/kernel/dtso.c
 create mode 100644 tools/testing/selftests/riscv/dtso/.gitignore
 create mode 100644 tools/testing/selftests/riscv/dtso/Makefile
 create mode 100644 tools/testing/selftests/riscv/dtso/dtso.c

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2.41.0