Re: [PATCH v4 1/2] x86/unwind: add ORC unwinder

[Josh Poimboeuf]

On Tue, Aug 08, 2017 at 01:09:08PM -0700, Andy Lutomirski wrote:
> >> c) just add ORC data for the alternative statically and _unconditionally_.
> >>
> >> No runtime registration. Just an unconditional entry for the
> >> particular IP that comes after the "pushfq". It cannot match the
> >> "callq" instruction, since it would be in the middle of that
> >> instruction.
> >>
> >> Basically, just do a "union" of the ORC data for all the alternatives.
> >>
> >> Now, objtool should still verify that the instruction pointers for
> >> alternatives are unique - or that they share the same ORC unwinder
> >> information if they are not.
> >>
> >> But in cases like this, when the instruction boundaires are different,
> >> things should "just work", with no need for any special cases.
> >>
> >> Hmm?
> >
> > Yeah, that might work.  Objtool already knows about alternatives, so it
> > might not be too hard.  I'll try it.
> 
> But this one's not an actual alternative, right?  It's a pv op.

Ah, right.  Objtool doesn't know about paravirt patching, unfortunately.

> I would advocate that we make it an alternative after all.  I frickin'
> hate the PV irq ops.  It would like roughly like this:
> 
> ALTERNATIVE "pushfq; popq %rax", "callq *pv_irq_ops.save_fl",
> X86_FEATURE_GODDAMN_PV_IRQ_OPS
> 
> (The obvious syntax error and the naming should probably be fixed.
> Also, this needs to live in an #ifdef because it needs to build on
> kernels with pv support.  It should also properly register itself as a
> pv patch site.)

Yeah, that would be really nice, assuming it's possible.  Otherwise I'll
need to teach objtool about the paravirt patches.

-- 
Josh