On 6/20/19 12:01 AM, Peter Zijlstra wrote: > > In particular we do not need the alignment. > > So what the x86 code does is: > > - overwrite the first byte of the instruction with a single byte trap > instruction > > - machine wide IPI which synchronizes I$ > > At this point, any CPU that encounters this instruction will trap; and > the trap handler will emulate the 'new' instruction -- typically a jump. > > - overwrite the tail of the instruction (if there is a tail) > > - machine wide IPI which syncrhonizes I$ > > At this point, nobody will execute the tail, because we'll still trap on > that first single byte instruction, but if they were to read the > instruction stream, the tail must be there. > > - overwrite the first byte of the instruction to now have a complete > instruction. > > - machine wide IPI which syncrhonizes I$ > > At this point, any CPU will encounter the new instruction as a whole, > irrespective of alignment. > > > So the benefit of this scheme is that is works irrespective of the > instruction fetch window size and don't need the 'funny' alignment > stuff. > > Now, I've no idea if something like this is feasible on ARC; for it to > work you need that 2 byte trap instruction -- since all instructions are > 2 byte aligned, you can always poke that without issue. We do have a 2 byte TRAP_S u6 which is used for all/any trap'ing: syscalls, software breakpoint, kprobes etc. But using it like x86 seems a bit excessive for ARC. Given that x86 doesn't implement flush_icache_range() it must have I$ snooping D$ and also this machine wide IPI sync I$ must be totally under the hood all hardware affair - unlike ARC which needs on_each_cpu( I$ line range). Using TRAP_S would actually requires 2 passes (and 2 rounds of IPI) for code patching - the last one to undo the TRAP_S itself. I do worry about the occasional alignment induced extra NOP_S instruction (2 byte) but there doesn't seem to be an easy solution. Heck if we could use the NOP_S / B_S in first place. While not a clean solution by any standards, could anything be done to reduce the code path of DO_ONCE() so that unlikely code is not too far off.
