[RFC V2 PATCH 0/9] Add support to handle misaligned accesses in S-mode

[Clément Léger]

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, SBI call for misaligned trap delegation
as well prctl support for PR_SET_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.

This series relies on 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] which allows to
either handles misaligned access in hardware or let the software do it.

With hardware misaligned access support:
$ ./spike --misaligned --initrd=rootfs.cpio \
  --kernel=arch/riscv/boot/Image opensbi_fw_jump.elf

Without hardware misaligned access support:
$ ./spike --initrd=rootfs.cpio --kernel=arch/riscv/boot/Image \
   opensbi_fw_jump.elf

[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

Changes in V2:
 - Fix prototypes declaration for handle_misaligned_load/store()
 - Fix build for !CONFIG_FPU
 - Added conditional build for various part of misaligned code handling
 - Added in-kernel misalignement fault support to be equivalent to
   existing SBI support
 - Added support for misalignment emulation detection for hwprobe
   reporting
 - Modified set/get_unaligned_ctl to use this detection of emulation
 - Added support for sysctl "unaligned_enabled"

Clément Léger (9):
  riscv: remove unused functions in traps_misaligned.c
  riscv: avoid missing prototypes warning
  riscv: add support for misaligned handling in S-mode
  riscv: report perf event for misaligned fault
  riscv: add support for sysctl unaligned_enabled control
  riscv: add support for SBI misalignment trap delegation
  riscv: report misaligned accesses emulation to hwprobe
  riscv: add support for PR_SET_UNALIGN and PR_GET_UNALIGN
  riscv: add floating point insn support to misaligned access emulation

 arch/riscv/Kconfig                    |   1 +
 arch/riscv/include/asm/cpufeature.h   |  10 +
 arch/riscv/include/asm/entry-common.h |   3 +
 arch/riscv/include/asm/processor.h    |   9 +
 arch/riscv/include/asm/sbi.h          |  11 +
 arch/riscv/kernel/Makefile            |   2 +-
 arch/riscv/kernel/fpu.S               | 117 +++++++++
 arch/riscv/kernel/process.c           |  18 ++
 arch/riscv/kernel/sbi.c               |  21 ++
 arch/riscv/kernel/setup.c             |   2 +
 arch/riscv/kernel/traps.c             |   9 -
 arch/riscv/kernel/traps_misaligned.c  | 352 ++++++++++++++++++++++----
 12 files changed, 498 insertions(+), 57 deletions(-)

-- 
2.40.1