Allison Husain
(replying to Saagar Jha)
@saagar in theory, doesn't FEAT_Debugv8p2 solve this whole mess? ESR.ISS.WPT reports the matched watchpoint number for a trap and is mandatory. Though, they kinda ruin it with WPTV which can make WPT invalid, but that goes away in 8p9 where WPT cannot be invalid.
https://arm.jonpalmisc.com/2023_09_sysreg/AArch64-esr_el2#fieldset_0-24_0_24-23_18-1
2 replies
Jason Molenda
(replying to Allison Husain)
@ezhes_ @saagar in streaming SVE mode a write near a watched region can trigger a false watchpoint hit, and that watchpoint number will be reported as the one hit. lldb changed its default watchpoint type to "modify" for write-wps last year, so we can ignore a false positive in this mode, and also because we use the MASK style wps to allow people to watch > 8byte blocks of memory on darwin, and we need to ignore writes outside of any non-pow-of-2 region they might request. e.g. "watch 96B”.
2 replies
Jason Molenda
(replying to Jason Molenda)
@ezhes_ @saagar should say - "modify watchpoint” in this context means “the memory being watched was mutated", so writing the same value there is not reported to the user. I think it's a more useful mode for who people use watchpoints tbh. A genuine write-watchpoint is still available if someone needs to audit every piece of code touching a region, even if storing the same value.
Jason Molenda
(replying to Jason Molenda)
@ezhes_ @saagar fun aside, lldb will also try to use multiple Byte Address Select watchpoints (1..8 bytes) on non-darwin AArch64 systems, e.g. Linux where only BAS watchpoints are available, or other targets like Intel. The engine that splits up a user's watchpoint request is flexible, but it only handles those two types of watchpoint capabilities today.