Re: [PATCH 3/6] x86/kvm/emulate: Avoid RET for fastops

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Apr 14, 2025 at 03:36:50PM -0700, Josh Poimboeuf wrote:
> On Mon, Apr 14, 2025 at 01:11:43PM +0200, Peter Zijlstra wrote:
> > Since there is only a single fastop() function, convert the FASTOP
> > stuff from CALL_NOSPEC+RET to JMP_NOSPEC+JMP, avoiding the return
> > thunks and all that jazz.
> > 
> > Specifically FASTOPs rely on the return thunk to preserve EFLAGS,
> > which not all of them can trivially do (call depth tracing suffers
> > here).
> > 
> > Objtool strenuously complains about things, therefore fix up the
> > various problems:
> > 
> >  - indirect call without a .rodata, fails to determine JUMP_TABLE,
> >    add an annotation for this.
> >  - fastop functions fall through, create an exception for this case
> >  - unreachable instruction after fastop_return, save/restore
> 
> I think this breaks unwinding.  Each of the individual fastops inherits
> fastop()'s stack but the ORC doesn't reflect that.

I'm not sure I understand. There is only the one location, and we
simply save/restore the state around the one 'call'.