linux-kernel - Re: [PATCH 4/7] objtool: Add support for return trampoline call

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Date:   Mon, 6 Apr 2020 11:31:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Julien Thierry <jthierry@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH 4/7] objtool: Add support for return trampoline call

On Mon, Apr 06, 2020 at 10:19:56AM +0200, Alexandre Chartre wrote:
> 
> On 4/4/20 5:51 PM, Peter Zijlstra wrote:
> > On Sat, Apr 04, 2020 at 09:22:32AM -0500, Josh Poimboeuf wrote:
> > > On Sat, Apr 04, 2020 at 03:32:18PM +0200, Peter Zijlstra wrote:
> > > > On Fri, Apr 03, 2020 at 05:46:20PM +0200, Peter Zijlstra wrote:
> > > > > On Fri, Apr 03, 2020 at 10:17:57AM -0500, Josh Poimboeuf wrote:
> > > > > > Peter, I think my previous idea for UNWIND_HINT_ADJUST stack_add=8 would
> > > > > > work here?
> > > > > 
> > > > > Yes, it would.
> > > > 
> > > > Sorry, I have reconsidered. While it will shut up objtool, it will not
> > > > 'work'. That is, the ORC data generated will not correctly unwind.
> > > > 
> > > > I'll try and write a longer email tonight.
> > > 
> > > Right, that's what I've been trying to say.  The ORC data will be
> > > non-deterministic unless we unroll the loop.  Or did you mean something
> > > else?
> > 
> > The below should result in deterministic code.
> > 
> > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > index 07e95dcb40ad..109ee65f4a11 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -59,8 +59,8 @@
> >   	jmp	775b;				\
> >   774:						\
> >   	dec	reg;				\
> > -	jnz	771b;				\
> > -	add	$(BITS_PER_LONG/8) * nr, sp;
> > +	add	$(BITS_PER_LONG/8) * $2, sp;	\
> > +	jnz	771b;
> >   #ifdef __ASSEMBLY__
> 
> Nice. This works fine and allows to remove ANNOTATE_NOSPEC_ALTERNATIVE when
> using __FILL_RETURN_BUFFER. However this is probably less performant because
> we now have nr/2 add instructions instead of just 1.

Does it actually matter though? That is, can you measure the difference?