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Date:   Wed, 04 Sep 2019 20:12:34 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH v3 2/3] Powerpc64/Watchpoint: Don't ignore extraneous
 exceptions
To:     mikey@neuling.org, mpe@ellerman.id.au,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc:     benh@kernel.crashing.org, christophe.leroy@c-s.fr,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        npiggin@gmail.com, paulus@samba.org
References: <20190710045445.31037-1-ravi.bangoria@linux.ibm.com>
        <20190710045445.31037-3-ravi.bangoria@linux.ibm.com>
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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