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

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Message-ID: <CAK9=C2X2J4ROJ7nKB0hcD9dVav7ZMFWw2Qy+GEsmYN_Tt8DxPQ@mail.gmail.com>
Date: Mon, 11 Aug 2025 15:11:27 +0530
From: Anup Patel <apatel@...tanamicro.com>
To: Jesse Taube <jesse@...osinc.com>
Cc: linux-riscv@...ts.infradead.org, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
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Subject: 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@...osinc.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@...osinc.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