[RFC,v2,0/4] RISC-V: Add dynamic TSO support

Message

Christoph Müllner Feb. 9, 2024, 6:41 a.m. UTC

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

This patch treats Ssdtso similar to TSO, i.e., the guest is always being
executed in TSO mode, which simplifies the implementation a lot while
still being correct. The downside is, that we have a performance penalty
on hosts that don't run with TSO.

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.

This series is based on the third draft of the Ssdtso specification
which can be found here:
  https://github.com/riscv/riscv-ssdtso/releases/tag/v1.0-draft3
Note, that the Ssdtso specification is in development state
(i.e., not frozen or even ratified) which is also the reason
why this series is marked as RFC.

This series saw the following changes since v1:
* Adding compatibility with Ztso (spec change in draft 3)
* Use PR_MEMORY_CONSISTENCY_MODEL* instead of numeric constants

This series is based on riscv-to-apply.next with two series on top:
* Sync hwprobe keys with kernel
  https://lists.gnu.org/archive/html/qemu-devel/2024-02/msg01293.html
* Add Ztso extension
  https://lists.gnu.org/archive/html/qemu-devel/2024-02/msg01296.html

This patchset can also be found in this GitHub branch:
  https://github.com/cmuellner/qemu/tree/ssdtso-v2

A Linux implementation of DTSO can be found in this GitHub branch:
  https://github.com/cmuellner/linux/tree/ssdtso-v2

Christoph Müllner (4):
  RISC-V: Add support for Ssdtso
  linux-user/riscv: Add Ssdtso extension to hwprobe
  linux-user/prctl: Add dynamic memory consistency model prctl API
  linux-user/riscv: Implement dynamic memory consistency model support

 linux-user/riscv/target_prctl.h | 76 ++++++++++++++++++++++++++++++++-
 linux-user/syscall.c            | 20 +++++++++
 target/riscv/cpu.c              | 13 +++++-
 target/riscv/cpu.h              |  1 +
 target/riscv/cpu_bits.h         |  3 ++
 target/riscv/cpu_cfg.h          |  1 +
 target/riscv/csr.c              | 14 ++++--
 target/riscv/translate.c        |  2 +-
 8 files changed, 123 insertions(+), 7 deletions(-)