Re: [PATCH v3 2/3] Powerpc64/Watchpoint: Don't ignore extraneous exceptions

From: Naveen N. Rao
Date: Wed Sep 04 2019 - 10:42:45 EST

Ravi Bangoria wrote:
On Powerpc64, watchpoint match range is double-word granular. On
a watchpoint hit, DAR is set to the first byte of overlap between
actual access and watched range. And thus it's quite possible that
DAR does not point inside user specified range. Ex, say user creates
a watchpoint with address range 0x1004 to 0x1007. So hw would be
configured to watch from 0x1000 to 0x1007. If there is a 4 byte
access from 0x1002 to 0x1005, DAR will point to 0x1002 and thus
interrupt handler considers it as extraneous, but it's actually not,
because part of the access belongs to what user has asked. So, let
kernel pass it on to user and let user decide what to do with it
instead of silently ignoring it. The drawback is, it can generate
false positive events.

I think you should do the additional validation here, instead of generating false positives. You should be able to read the instruction, run it through analyse_instr(), and then use OP_IS_LOAD_STORE() and GETSIZE() to understand the access range. This can be used to then perform a better match against what the user asked for.

- Naveen