arm64: debug: split hardware watchpoint exception entry

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@arm.com>
Tested-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Reviewed-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20250707114109.35672-11-ada.coupriediaz@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/arch/arm64/include/asm/exception.h b/arch/arm64/include/asm/exception.h
index c8e7c61b8ac4..0362fecc5f69 100644
--- a/arch/arm64/include/asm/exception.h
+++ b/arch/arm64/include/asm/exception.h
@@ -63,8 +63,12 @@ void do_debug_exception(unsigned long addr_if_watchpoint, unsigned long esr,
 			struct pt_regs *regs);
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
 void do_breakpoint(unsigned long esr, struct pt_regs *regs);
+void do_watchpoint(unsigned long addr, unsigned long esr,
+			struct pt_regs *regs);
 #else
 static inline void do_breakpoint(unsigned long esr, struct pt_regs *regs) {}
+static inline void do_watchpoint(unsigned long addr, unsigned long esr,
+			struct pt_regs *regs) {}
 #endif /* CONFIG_HAVE_HW_BREAKPOINT */
 void do_el0_softstep(unsigned long esr, struct pt_regs *regs);
 void do_el1_softstep(unsigned long esr, struct pt_regs *regs);