From 07c1c91de384f8fa7cad88acc71ac6a7b33cefcf Mon Sep 17 00:00:00 2001 From: Andrew Burgess Date: Tue, 28 Mar 2023 11:24:58 +0100 Subject: gdb: warn when converting h/w watchpoints to s/w On amd64 (at least) if a user sets a watchpoint before the inferior has started then GDB will assume that a hardware watchpoint can be created. When the inferior starts there is a chance that the watchpoint can't actually be create as a hardware watchpoint, in which case (currently) GDB will silently convert the watchpoint to a software watchpoint. Here's an example session: (gdb) p sizeof var $1 = 4000 (gdb) watch var Hardware watchpoint 1: var (gdb) info watchpoints Num Type Disp Enb Address What 1 hw watchpoint keep y var (gdb) starti Starting program: /home/andrew/tmp/watch Program stopped. 0x00007ffff7fd3110 in _start () from /lib64/ld-linux-x86-64.so.2 (gdb) info watchpoints Num Type Disp Enb Address What 1 watchpoint keep y var (gdb) Notice that before the `starti` command the watchpoint is showing as a hardware watchpoint, but afterwards it is showing as a software watchpoint. Additionally, note that we clearly told the user we created a hardware watchpoint: (gdb) watch var Hardware watchpoint 1: var I think this is bad. I used `starti`, but if the user did `start` or even `run` then the inferior is going to be _very_ slow, which will be unexpected -- after all, we clearly told the user that we created a hardware watchpoint, and the manual clearly says that hardware watchpoints are fast (at least compared to s/w watchpoints). In this patch I propose adding a new warning which will be emitted when GDB downgrades a h/w watchpoint to s/w. The session now looks like this: (gdb) p sizeof var $1 = 4000 (gdb) watch var Hardware watchpoint 1: var (gdb) info watchpoints Num Type Disp Enb Address What 1 hw watchpoint keep y var (gdb) starti Starting program: /home/andrew/tmp/watch warning: watchpoint 1 downgraded to software watchpoint Program stopped. 0x00007ffff7fd3110 in _start () from /lib64/ld-linux-x86-64.so.2 (gdb) info watchpoints Num Type Disp Enb Address What 1 watchpoint keep y var (gdb) The important line is: warning: watchpoint 1 downgraded to software watchpoint It's not much, but hopefully it will be enough to indicate to the user that something unexpected has occurred, and hopefully, they will not be surprised when the inferior runs much slower than they expected. I've added an amd64 only test in gdb.arch/, I didn't want to try adding this as a global test as other architectures might be able to support the watchpoint request in h/w. Also the test is skipped for extended-remote boards as there's a different set of options for limiting hardware watchpoints on remote targets, and this test isn't about them. Reviewed-By: Lancelot Six --- gdb/breakpoint.c | 19 +++++++++++++++++-- 1 file changed, 17 insertions(+), 2 deletions(-) (limited to 'gdb/breakpoint.c') diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c index ebe9794..47f35c6 100644 --- a/gdb/breakpoint.c +++ b/gdb/breakpoint.c @@ -2143,6 +2143,21 @@ update_watchpoint (struct watchpoint *b, bool reparse) } } + /* Helper function to bundle possibly emitting a warning along with + changing the type of B to bp_watchpoint. */ + auto change_type_to_bp_watchpoint = [] (breakpoint *bp) + { + /* Only warn for breakpoints that have been assigned a +ve number, + anything else is either an internal watchpoint (which we don't + currently create) or has not yet been finalized, in which case + this change of type will be occurring before the user is told + the type of this watchpoint. */ + if (bp->type == bp_hardware_watchpoint && bp->number > 0) + warning (_("watchpoint %d downgraded to software watchpoint"), + bp->number); + bp->type = bp_watchpoint; + }; + /* Change the type of breakpoint between hardware assisted or an ordinary watchpoint depending on the hardware support and free hardware slots. Recheck the number of free hardware slots @@ -2200,7 +2215,7 @@ update_watchpoint (struct watchpoint *b, bool reparse) "resources for this watchpoint.")); /* Downgrade to software watchpoint. */ - b->type = bp_watchpoint; + change_type_to_bp_watchpoint (b); } else { @@ -2221,7 +2236,7 @@ update_watchpoint (struct watchpoint *b, bool reparse) "read/access watchpoint.")); } else - b->type = bp_watchpoint; + change_type_to_bp_watchpoint (b); loc_type = (b->type == bp_watchpoint? bp_loc_software_watchpoint : bp_loc_hardware_watchpoint); -- cgit v1.1