Re: [PATCH 5/8] riscv: hw_breakpoint: Use icount for single stepping

Hi Jesse,

On Wed, Aug 6, 2025 at 1:10 AM Jesse Taube jesse@rivosinc.com wrote:

The Sdtrig RISC-V ISA extension does not have a resume flag for returning to and executing the instruction at the breakpoint. To avoid skipping the instruction or looping, it is necessary to remove the hardware breakpoint and single step. Use the icount feature of Sdtrig to accomplish this. Use icount as default with an option to allow software-based single stepping when hardware or SBI does not have icount functionality, as it may cause unwanted side effects when reading the instruction from memory.

Signed-off-by: Jesse Taube jesse@rivosinc.com

OpenSBI implementation of sbi_debug_read_triggers does not return the updated CSR values. There needs to be a check for working sbi_debug_read_triggers before this works.

https://lists.riscv.org/g/tech-prs/message/1476

RFC -> V1:

• Add dbtr_mode to rv_init_icount_trigger
• Add icount_triggered to check which breakpoint was triggered
• Fix typo: s/affects/effects
• Move HW_BREAKPOINT_COMPUTE_STEP to Platform type

---

arch/riscv/Kconfig | 11 ++ arch/riscv/kernel/hw_breakpoint.c | 179 +++++++++++++++++++++++++++--- 2 files changed, 172 insertions(+), 18 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig index fd8b62cdc6f5..37f01ed199f3 100644 --- a/arch/riscv/Kconfig +++ b/arch/riscv/Kconfig @@ -546,6 +546,17 @@ config RISCV_COMBO_SPINLOCKS

endchoice

+config HW_BREAKPOINT_COMPUTE_STEP

•   bool "Allow computing hardware breakpoint step address"
  

•   default n
  

•   depends on HAVE_HW_BREAKPOINT
  

•   help
  

•     Select this option if hardware breakpoints are desired, but
  

•     hardware or SBI does not have icount functionality. This may cause
  

•     unwanted side effects when reading the instruction from memory.
  

•     If unsure, say N.
  

We expect the same kernel image to work on a platform with icount triggers and without icount triggers.

Please drop this kconfig option. The decision of falling back to computing hardware breakpoint step address should be at boot-time and not compile-time.

Regards, Anup