Re: [PATCH v4 12/13] arm64: debug: split bkpt32 exception entry

[Will Deacon <will@kernel.org>]

On Fri, Jun 20, 2025 at 10:12:06PM +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.
> 
> The BKPT32 exception can only be triggered by a BKPT instruction. Thus,
> we know that the PC is a legitimate address and isn't being used to train
> a branch predictor with a bogus address : we don't need to call
> `arm64_apply_bp_hardening()`.
> 
> The handler for this exception only pends a signal and doesn't depend
> on any per-CPU state : we don't need to inhibit preemption, nor do we
> need to keep the DAIF exceptions masked, so we can unmask them earlier.
> 
> Split the BKPT32 exception entry and adjust function signatures and its
> behaviour to match its relaxed constraints compared to other
> debug exceptions.
> We can also remove `NOKRPOBE_SYMBOL`, as this cannot lead to a kprobe
> recursion.
> 
> This replaces the last usage of `el0_dbg()`, so remove it.
> 
> Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> Tested-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
> ---
>  arch/arm64/include/asm/exception.h |  1 +
>  arch/arm64/kernel/debug-monitors.c |  7 +++++++
>  arch/arm64/kernel/entry-common.c   | 21 +++++++++------------
>  3 files changed, 17 insertions(+), 12 deletions(-)

Reviewed-by: Will Deacon <will@kernel.org>

Will