[PATCH v5 10/13] arm64: debug: split hardware watchpoint exception entry

[Will Deacon]

On Fri, Jun 27, 2025 at 06:20:35PM +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.
> 
> As the hardware watchpoint handler only returns 0 and never triggers
> the call to `arm64_notify_die()`, we can call it directly from
> `entry-common.c`.
> 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 at arm.com>
> Tested-by: Luis Claudio R. Goncalves <lgoncalv at redhat.com>
> Reviewed-by: Will Deacon <will at kernel.org>
> ---
>  arch/arm64/include/asm/exception.h |  4 ++++
>  arch/arm64/kernel/entry-common.c   | 31 +++++++++++++++++++++++++++++-
>  arch/arm64/kernel/hw_breakpoint.c  | 17 +++++-----------
>  3 files changed, 39 insertions(+), 13 deletions(-)

Not a huge deal, but I just noticed that this patch doesn't build on its
own and so this series isn't bisectable.

> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 7265bef96672..c2ea1653dbb8 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -553,10 +553,20 @@ 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)
>  {
> +	/* Watchpoints are the only debug exception to write FAR_EL1 */
>  	unsigned long far = read_sysreg(far_el1);
>  
> +	arm64_enter_el1_dbg(regs);
> +	debug_exception_enter(regs);
> +	do_watchpoint(far, esr, regs);
> +	debug_exception_exit(regs);
> +	arm64_exit_el1_dbg(regs);
> +}
> +
> +static void noinstr el1_dbg(struct pt_regs *regs, unsigned long esr)
> +{
>  	arm64_enter_el1_dbg(regs);
>  	if (!cortex_a76_erratum_1463225_debug_handler(regs))
>  		do_debug_exception(far, esr, regs);

This leads to:

Failed to build patch #10: 0d0a8f024a19 arm64: debug: split hardware watchpoint exception entry
arch/arm64/kernel/entry-common.c: In function ‘el1_dbg’:
arch/arm64/kernel/entry-common.c:572:36: error: ‘far’ undeclared (first use in this function)
  572 |                 do_debug_exception(far, esr, regs);
      |                                    ^~~


Catalin -- do you reckon we should fix this? It's only this series
sitting on for-next/debug-entry, nobody has pulled it afaik and we still
have plenty of time before the merge window so a rebase probably won't
hurt anybody.

Will