Re: [PATCH v2 08/11] arm64: debug: split hardware watchpoint exception entry

[Will Deacon <will@kernel.org>]

On Mon, May 12, 2025 at 06:43:23PM +0100, Ada Couprie Diaz wrote:
> Currently all debug exceptions share common entry code and are routed
> to `do_debug_exception()`, which calls dynamically-registered
> handlers for each specific debug exception. This is unfortunate as
> different debug exceptions have different entry handling requirements,
> and it would be better to handle these distinct requirements earlier.
> 
> Hardware watchpoints are the only debug exceptions that will write
> FAR_EL1, so we need to preserve it and pass it down.
> However, they cannot be used to maliciously train branch predictors, so
> we can omit calling `arm64_bp_hardening()`, nor do they need to handle
> the Cortex-A76 erratum #1463225, as it only applies to single stepping
> exceptions.
> 
> Split the hardware watchpoint exception entry and adjust the behaviour
> to match the lack of needed mitigations.
> 
> Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> ---
>  arch/arm64/include/asm/exception.h |  2 ++
>  arch/arm64/kernel/entry-common.c   | 31 +++++++++++++++++++++++++++++-
>  arch/arm64/kernel/hw_breakpoint.c  | 14 ++++++++++----
>  3 files changed, 42 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/exception.h b/arch/arm64/include/asm/exception.h
> index cbcd832bf58e..eb465c375d34 100644
> --- a/arch/arm64/include/asm/exception.h
> +++ b/arch/arm64/include/asm/exception.h
> @@ -63,6 +63,8 @@ void do_debug_exception(unsigned long addr_if_watchpoint, unsigned long esr,
>  			struct pt_regs *regs);
>  void do_breakpoint(unsigned long esr, struct pt_regs *regs);
>  void do_softstep(unsigned long esr, struct pt_regs *regs);
> +void do_watchpoint(unsigned long addr, unsigned long esr,
> +			struct pt_regs *regs);
>  void do_fpsimd_acc(unsigned long esr, struct pt_regs *regs);
>  void do_sve_acc(unsigned long esr, struct pt_regs *regs);
>  void do_sme_acc(unsigned long esr, struct pt_regs *regs);
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 8814ad24e707..6e70130d2741 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -530,10 +530,20 @@ static void noinstr el1_softstp(struct pt_regs *regs, unsigned long esr)
>  	arm64_exit_el1_dbg(regs);
>  }
>  
> -static void noinstr el1_dbg(struct pt_regs *regs, unsigned long esr)
> +static void noinstr el1_watchpt(struct pt_regs *regs, unsigned long esr)
>  {
> +	/* Only watchpoints write FAR_EL1 */

nit: But maybe scope the comment (here and in the el0 handler) for debug
exceptions?
e.g.

	/* Watchpoints are the only debug exception to write FAR_EL1 */

?

Will