linux-kernel - [PATCH 0/7] Add support to handle misaligned accesses in S-mode

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Date:   Tue, 26 Sep 2023 17:03:09 +0200
From:   Clément Léger <cleger@...osinc.com>
To:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>
Cc:     Clément Léger <cleger@...osinc.com>,
        Atish Patra <atishp@...osinc.com>,
        Andrew Jones <ajones@...tanamicro.com>,
        Evan Green <evan@...osinc.com>,
        Björn Topel <bjorn@...osinc.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Ron Minnich <rminnich@...il.com>,
        Daniel Maslowski <cyrevolt@...glemail.com>
Subject: [PATCH 0/7] Add support to handle misaligned accesses in S-mode

Since commit 61cadb9 ("Provide new description of misaligned load/store
behavior compatible with privileged architecture.") in the RISC-V ISA
manual, it is stated that misaligned load/store might not be supported.
However, the RISC-V kernel uABI describes that misaligned accesses are
supported. In order to support that, this series adds support for S-mode
handling of misaligned accesses as well support for prctl(PR_UNALIGN).

Handling misaligned access in kernel allows for a finer grain control
of the misaligned accesses behavior, and thanks to the prctl call, can
allow disabling misaligned access emulation to generate SIGBUS. User
space can then optimize its software by removing such access based on
SIGBUS generation.

Currently, this series is useful for people that uses a SBI that does
not handled misaligned traps. In a near future, this series will make
use a SBI extension [1] allowing to request delegation of the
misaligned load/store traps to the S-mode software. This extension has
been submitted for review to the riscv tech-prs group. An OpenSBI
implementation for this spec is available at [2].

This series can be tested using the spike simulator [3] and an openSBI
version [4] which allows to always delegate misaligned load/store to
S-mode.

[1] https://lists.riscv.org/g/tech-prs/message/540
[2] https://github.com/rivosinc/opensbi/tree/dev/cleger/fw_feature_upstream
[3] https://github.com/riscv-software-src/riscv-isa-sim
[4] https://github.com/rivosinc/opensbi/tree/dev/cleger/no_misaligned

Clément Léger (7):
  riscv: remove unused functions in traps_misaligned.c
  riscv: add support for misaligned handling in S-mode
  riscv: report perf event for misaligned fault
  riscv: add floating point insn support to misaligned access emulation
  riscv: add support for sysctl unaligned_enabled control
  riscv: report misaligned accesses emulation to hwprobe
  riscv: add support for PR_SET_UNALIGN and PR_GET_UNALIGN

 arch/riscv/Kconfig                    |   1 +
 arch/riscv/include/asm/cpufeature.h   |   6 +
 arch/riscv/include/asm/entry-common.h |   3 +
 arch/riscv/include/asm/processor.h    |   9 +
 arch/riscv/kernel/Makefile            |   2 +-
 arch/riscv/kernel/cpufeature.c        |   6 +-
 arch/riscv/kernel/fpu.S               | 117 ++++++++
 arch/riscv/kernel/process.c           |  18 ++
 arch/riscv/kernel/setup.c             |   1 +
 arch/riscv/kernel/traps.c             |   9 -
 arch/riscv/kernel/traps_misaligned.c  | 374 ++++++++++++++++++++++----
 11 files changed, 488 insertions(+), 58 deletions(-)

-- 
2.40.1