Messages in this thread | | | Date | Mon, 10 Dec 2012 14:56:58 +0000 | From | Will Deacon <> | Subject | Re: [PATCH] ARM: ftrace: Ensure code modifications are synchronised across all cpus |
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Hi Jamie,
Thanks for summarising the thread so far.
On Mon, Dec 10, 2012 at 01:40:01PM +0000, Jamie Lokier wrote: > On Fri, 2012-12-07 at 19:02 +0000, Will Deacon wrote: > > For ARMv7, there are small subsets of instructions for ARM and Thumb which > > are guaranteed to be atomic wrt concurrent modification and execution of > > the instruction stream between different processors: > > > > Thumb: The 16-bit encodings of the B, NOP, BKPT, and SVC instructions. > > ARM: The B, BL, NOP, BKPT, SVC, HVC, and SMC instructions. > > Thumb 32-bit ftrace call isn't in the above list. > > Questions: does the above concurrent modification guarantee require > both the old instruction _and_ the new one to be among those listed, > or is it enough to be just the new one (for example when setting a > normal software breakpoint, that would be useful)? Can it be the old > one and not the new (for example when removing a software breakpoint, > that would be useful)? Does that subset mean replacing any of the > listed instructions by any of the others is ok, or any of the listed > with another of the same type?
Yes, the target instruction also has to be listed. However, I only described the simple cases above... there are a number of exceptions when it comes to 32-bit Thumb-2 encodings of BL (I hinted at one of them before):
- The two halfwords of a 32-bit BL instruction can each be replaced with the relevant halfword from another BL instruction. This basically means you can change the immediate fields, as I mentioned earlier.
- The most-significant halfword of a 32-bit BL instruction can be replaced with B, BKPT or SVC (i.e. not NOP).
- A 16-bit B, BKPT, or SVC instruction can be replaced with the most-significant halfword of a BL instruction.
The latter points mean we can effectively nop out bl instructions, and put them back again.
> This is what makes me wonder, if it's safe to replace the 32-bit > mcount call with a 16-bit short jump: > > > On Mon, Dec 10, 2012 at 11:04:05AM +0000, Jon Medhurst (Tixy) wrote: > > > So this means for things like kprobes which can modify arbitrary kernel > > > code we are going to need to continue to always use some form of > > > stop_the_whole_system() function? > > > > > > Also, kprobes currently uses patch_text() which only uses stop_machine > > > for Thumb2 instructions which straddle a word boundary, so this needs > > > changing? > > Will Deacon replied: > > Yes; if you're modifying instructions other than those mentioned above, then > > you'll need to synchronise the CPUs, update the instructions, perform > > cache-maintenance on the writing CPU and then execute an isb on the > > executing core (this last bit isn't needed if you're going to go through an > > exception return to get back to the new code -- depends on how your > > stop/resume code works). > > If I've understood that exchange, it implies that using patch_text() > to replace an instruction not in the list of special ones, with a trap > or jump, isn't ok? And so it's ok to replace the NOP with a short > branch (since 16-bit "B" is in the list), but it's not ok to replace > 16-bit "B" with the 32-bit ftrace call; and the same going the other way?
Well, these restrictions only apply if you want to avoid synchronising the CPUs. Hopefully my explanation above answers your questions!
Will
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