Re: [PATCH] ARC: ARCv2: jump label: implement jump label patching

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 20, 2019 at 11:48:17AM -0700, Vineet Gupta wrote:
> 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).

I always forget the exact details, but we do have to execute what is
called a serializing instruction to flush CPU state and force it to
re-read the actual instructions -- see sync_core().

> 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.

Correct -- we do 3, like detailed in the other email. But we figured the
actual poking of text is the slow path anyway.

> 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.

if one could somehow get the arch_static_branch*() things to
conditionally emit either the 2 or 4 byte jump, depending on the offset
(which is known there, since we stick it in the __jump_table), then we
can have arch_jump_label_transform() use that same condition to switch
between 2 and 4 bytes too.

I just don't know if it's possible :-/