Messages in this thread | | | Date | Mon, 10 Feb 2014 15:24:52 +0000 | From | Russell King - ARM Linux <> | Subject | Re: [RFC PATCH] ARM: Add imprecise abort enable/disable macro |
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On Mon, Feb 10, 2014 at 03:12:47PM +0000, Dave Martin wrote: > Firstly, blindly adding 4 to PC is obviouly not right, partly because we > might be running an unrelated thread by the time the abort fires, and > also because the affected instruction might not be 4 bytes in size in a > Thumb kernel.
Exactly. We ended up on some platforms having special accessors for PCI where we included a number of 'mov r0, r0' instructions after the accessor so we could properly cope with them - but this required knowledge that we were going to only receive an imprecise abort from these accessors and only for a few cycles after the instruction.
However, that's not true with modern architectures. The point they're received will _not_ be the load/store which resulted in the abort, and in the case of a write, they could be many hundreds of cycles later, especially if the write has been buffered.
So adding four to the PC is definitely a very /bad/ thing to do.
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