Messages in this thread | | | Date | Sun, 21 Oct 2012 20:15:53 +0200 | From | Rabin Vincent <> | Subject | Re: [PATCH 3/9] uprobes: allow ignoring of probe hits |
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On Wed, Oct 17, 2012 at 07:35:10PM +0200, Oleg Nesterov wrote: > On 10/16, Rabin Vincent wrote: > > 2012/10/15 Oleg Nesterov <oleg@redhat.com>: > > > Not sure I understand why we shouldn't call handlers in this case, > > > but OK, I know nothing about arm. > > > > This old discussion about kprobes should be useful: > > > > http://lists.infradead.org/pipermail/linux-arm-kernel/2011-March/045755.html > > Thanks... Not sure I understand this discussion... > > And, to clarify, I am not arguing. Just curious. > > So, is this like cmov on x86? And this patch allows to not report if > the condition is not true? Or there are other issues on arm?
Yes, I guess this is like CMOV on x86. In the ARM instruction set most instructions can be conditionally executed.
In order to set the probe on a conditional instruction, we use an undefined instruction with the same condition as the instruction we replace. However, it is implementation defined whether an undefined instruction with a failing condition code will trigger an undefined instruction exception or just be executed as a NOP. So for those processor implementations where we do get the undefined instruction exception even for a failing condition code, we have to ignore it in order to provide consistent behaviour.
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