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Add comment describing continuable/steppable/non-steppable watchpoints

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These weren't described anywhere in the sources.

gdb/ChangeLog:
2018-08-31  Pedro Alves  <palves@redhat.com>

	* gdbarch.sh (have_nonsteppable_watchpoint): Add comment.
	* target.h (Hardware watchpoint interfaces): Describe
	continuable/steppable/non-steppable watchpoints.
	* gdbarch.h, gdbarch.c: Regenerate.
This commit is contained in:
Pedro Alves 2018-08-31 14:24:13 +01:00
parent 7eb65fafed
commit 7ea65f08fa
4 changed files with 45 additions and 0 deletions
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7
gdb/ChangeLog
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@ -1,3 +1,10 @@
2018-08-31 Pedro Alves <palves@redhat.com>
* gdbarch.sh (have_nonsteppable_watchpoint): Add comment.
* target.h (Hardware watchpoint interfaces): Describe
continuable/steppable/non-steppable watchpoints.
* gdbarch.h, gdbarch.c: Regenerate.
2018-08-31 Pedro Alves <palves@redhat.com>
* nto-procfs.c (nto_procfs_target::have_continuable_watchpoint):

2
gdb/gdbarch.h
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@ -818,6 +818,8 @@ extern void set_gdbarch_adjust_dwarf2_line (struct gdbarch *gdbarch, gdbarch_adj
extern int gdbarch_cannot_step_breakpoint (struct gdbarch *gdbarch);
extern void set_gdbarch_cannot_step_breakpoint (struct gdbarch *gdbarch, int cannot_step_breakpoint);
/* See comment in target.h about continue, steppable and non-steppable watchpoints. */
extern int gdbarch_have_nonsteppable_watchpoint (struct gdbarch *gdbarch);
extern void set_gdbarch_have_nonsteppable_watchpoint (struct gdbarch *gdbarch, int have_nonsteppable_watchpoint);

2
gdb/gdbarch.sh
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@ -707,6 +707,8 @@ f;CORE_ADDR;adjust_dwarf2_addr;CORE_ADDR pc;pc;;default_adjust_dwarf2_addr;;0
# stop PC.
f;CORE_ADDR;adjust_dwarf2_line;CORE_ADDR addr, int rel;addr, rel;;default_adjust_dwarf2_line;;0
v;int;cannot_step_breakpoint;;;0;0;;0
# See comment in target.h about continuable, steppable and
# non-steppable watchpoints.
v;int;have_nonsteppable_watchpoint;;;0;0;;0
F;int;address_class_type_flags;int byte_size, int dwarf2_addr_class;byte_size, dwarf2_addr_class
M;const char *;address_class_type_flags_to_name;int type_flags;type_flags

34
gdb/target.h
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@ -1905,6 +1905,40 @@ extern struct thread_info *target_thread_handle_to_thread_info
/* Hardware watchpoint interfaces. */
/* GDB's current model is that there are three "kinds" of watchpoints,
with respect to when they trigger and how you can move past them.
Those are: continuable, steppable, and non-steppable.
Continuable watchpoints are like x86's -- those trigger after the
memory access's side effects are fully committed to memory. I.e.,
they trap with the PC pointing at the next instruction already.
Continuing past such a watchpoint is doable by just normally
continuing, hence the name.
Both steppable and non-steppable watchpoints trap before the memory
access. I.e, the PC points at the instruction that is accessing
the memory. So GDB needs to single-step once past the current
instruction in order to make the access effective and check whether
the instruction's side effects change the watched expression.
Now, in order to step past that instruction, depending on
architecture and target, you can have two situations:
- steppable watchpoints: you can single-step with the watchpoint
still armed, and the watchpoint won't trigger again.
- non-steppable watchpoints: if you try to single-step with the
watchpoint still armed, you'd trap the watchpoint again and the
thread wouldn't make any progress. So GDB needs to temporarily
remove the watchpoint in order to step past it.
If your target/architecture does not signal that it has either
steppable or non-steppable watchpoints via either
target_have_steppable_watchpoint or
gdbarch_have_nonsteppable_watchpoint, GDB assumes continuable
watchpoints. */
/* Returns non-zero if we were stopped by a hardware watchpoint (memory read or
write). Only the INFERIOR_PTID task is being queried. */