Re: [PATCH 7/9] x86/unwind/orc: Fall back to using frame pointers for generated code

[Josh Poimboeuf]

On Thu, Jun 13, 2019 at 08:58:48PM -0500, Josh Poimboeuf wrote:
> On Thu, Jun 13, 2019 at 06:42:45PM -0700, Alexei Starovoitov wrote:
> > On Thu, Jun 13, 2019 at 08:30:51PM -0500, Josh Poimboeuf wrote:
> > > On Thu, Jun 13, 2019 at 03:00:55PM -0700, Alexei Starovoitov wrote:
> > > > > @@ -392,8 +402,16 @@ bool unwind_next_frame(struct unwind_state *state)
> > > > >  	 * calls and calls to noreturn functions.
> > > > >  	 */
> > > > >  	orc = orc_find(state->signal ? state->ip : state->ip - 1);
> > > > > -	if (!orc)
> > > > > -		goto err;
> > > > > +	if (!orc) {
> > > > > +		/*
> > > > > +		 * As a fallback, try to assume this code uses a frame pointer.
> > > > > +		 * This is useful for generated code, like BPF, which ORC
> > > > > +		 * doesn't know about.  This is just a guess, so the rest of
> > > > > +		 * the unwind is no longer considered reliable.
> > > > > +		 */
> > > > > +		orc = &orc_fp_entry;
> > > > > +		state->error = true;
> > > > 
> > > > That seems fragile.
> > > 
> > > I don't think so.  The unwinder has sanity checks to make sure it
> > > doesn't go off the rails.  And it works just fine.  The beauty is that
> > > it should work for all generated code (not just BPF).
> > > 
> > > > Can't we populate orc_unwind tables after JIT ?
> > > 
> > > As I mentioned it would introduce a lot more complexity.  For each JIT
> > > function, BPF would have to tell ORC the following:
> > > 
> > > - where the BPF function lives
> > > - how big the stack frame is
> > > - where RBP and other callee-saved regs are on the stack
> > 
> > that sounds like straightforward addition that ORC should have anyway.
> > right now we're not using rbp in the jit-ed code,
> > but one day we definitely will.
> > Same goes for r12. It's reserved right now for 'strategic use'.
> > We've been thinking to add another register to bpf isa.
> > It will map to r12 on x86. arm64 and others have plenty of regs to use.
> > The programs are getting bigger and register spill/fill starting to
> > become a performance concern. Extra register will give us more room.
> 
> With CONFIG_FRAME_POINTER, RBP isn't available.  If you look at all the
> code in the entire kernel you'll notice that BPF JIT is pretty much the
> only one still clobbering it.

Hm.  If you wanted to eventually use R12 for other purposes, there might
be a way to abstract BPF_REG_FP such that it doesn't actually need a
dedicated register.  The BPF program's frame pointer will always be a
certain constant offset away from RBP (real frame pointer), so accesses
to BPF_REG_FP could still be based on RBP, but with an offset added to
it.

-- 
Josh