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

[Josh Poimboeuf]

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.

Should they just be moved to a proper .S file?

-- 
Josh