[PATCH 1/7] powerpc/watchpoints: Explain thread_change_pc() more

Benjamin Gray bgray at linux.ibm.com
Tue Aug 1 11:17:38 AEST 2023

The behaviour of the thread_change_pc() function is a bit cryptic
without being more familiar with how the watchpoint logic handles
perf's after-execute semantics.

Expand the comment to explain why we can re-insert the breakpoint and
unset the perf_single_step flag.

Signed-off-by: Benjamin Gray <bgray at linux.ibm.com>
 arch/powerpc/kernel/hw_breakpoint.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
index e1b4e70c8fd0..bad2991f906b 100644
--- a/arch/powerpc/kernel/hw_breakpoint.c
+++ b/arch/powerpc/kernel/hw_breakpoint.c
@@ -499,6 +499,10 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
  * Restores the breakpoint on the debug registers.
  * Invoke this function if it is known that the execution context is
  * about to change to cause loss of MSR_SE settings.
+ *
+ * The perf watchpoint will simply re-trigger once the thread is started again,
+ * and the watchpoint handler will set up MSR_SE and perf_single_step as
+ * needed.
 void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs)

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