[0/8] riscv: ftrace: atmoic patching and preempt improvements

Message

Andy Chiu June 13, 2024, 7:11 a.m. UTC

This series makes atmoic code patching possible in riscv ftrace. A
direct benefit of this is that we can get rid of stop_machine() when
patching function entries. This also makes it possible to run ftrace
with full kernel preemption. Before this series, the kernel initializes
patchable function entries to NOP4 + NOP4. To start tracing, it updates
entries to AUIPC + JALR while holding other cores in stop_machine.
stop_machine() is required because it is impossible to update 2
instructions, and be seen atomically. And preemption must have to be
prevented, as kernel preemption allows process to be scheduled out while
executing on one of these instruction pairs.

This series addresses the problem by initializing the first NOP4 to
AUIPC. So, atmoic patching is possible because the kernel only has to
update one instruction. As long as the instruction is naturally aligned,
then it is expected to be updated atomically.

However, the address range of the ftrace trampoline is limited to +-2K
from ftrace_caller after appplying this series. This issue is expected
to be solved by Puranjay's CALL_OPS, where it adds 8B naturally align
data in front of pacthable functions and can  use it to direct execution
out to any custom trampolines.

The series is composed by two parts. The first part (1-3) cleans up
existing issues that was found during testing of and not caused by the
implementation. The second part modifies the ftrace code patching
mechanism (4-6) as mentioned above. Then prepare ftrace to be able to
run with kernel preemption (7,8)

---
Andy Chiu (8):
      riscv: stacktrace: convert arch_stack_walk() to noinstr
      tracing: do not trace kernel_text_address()
      riscv: ftrace: support fastcc in Clang for WITH_ARGS
      riscv: ftrace: align patchable functions to 4 Byte boundary
      riscv: ftrace: prepare ftrace for atomic code patching
      riscv: ftrace: do not use stop_machine to update code
      riscv: vector: Support calling schedule() for preemptible Vector
      riscv: ftrace: support PREEMPT

 arch/riscv/Kconfig                 |   3 +-
 arch/riscv/Makefile                |   7 +-
 arch/riscv/include/asm/ftrace.h    |  11 +++
 arch/riscv/include/asm/processor.h |   5 ++
 arch/riscv/include/asm/vector.h    |  22 +++++-
 arch/riscv/kernel/asm-offsets.c    |   7 ++
 arch/riscv/kernel/ftrace.c         | 133 ++++++++++++++++---------------------
 arch/riscv/kernel/mcount-dyn.S     |  25 +++++--
 arch/riscv/kernel/stacktrace.c     |   2 +-
 kernel/extable.c                   |   4 +-
 10 files changed, 129 insertions(+), 90 deletions(-)
---
base-commit: 1613e604df0cd359cf2a7fbd9be7a0bcfacfabd0
change-id: 20240613-dev-andyc-dyn-ftrace-v4-941d4a00ea19

Best regards,