linux-kernel - Re: [PATCH V2 9/9] x86/speculation: Remove all ANNOTATE_NOSPEC

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Date:   Tue, 7 Apr 2020 11:28:38 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, jthierry@...hat.com,
        tglx@...utronix.de
Subject: Re: [PATCH V2 9/9] x86/speculation: Remove all
 ANNOTATE_NOSPEC_ALTERNATIVE directives

On Tue, Apr 07, 2020 at 06:18:51PM +0200, Alexandre Chartre wrote:
> 
> On 4/7/20 4:32 PM, Alexandre Chartre wrote:
> > 
> > On 4/7/20 3:34 PM, Josh Poimboeuf wrote:
> > > On Tue, Apr 07, 2020 at 03:28:37PM +0200, Peter Zijlstra wrote:
> > > > Josh, we should probably have objtool verify it doesn't emit ORC entries
> > > > in alternative ranges.
> > > 
> > > Agreed, it might be as simple as checking for insn->alt_group in the
> > > INSN_STACK check or in update_insn_state().
> > > 
> > 
> > We could do that only for the "objtool orc generate" command. That way
> > "objtool check" would still check the alternative, but "objtool orc generate"
> > will just use the first half of the alternative (like it does today with
> > ANNOTATE_NOSPEC_ALTERNATIVE). We can even keep all ANNOTATE_NOSPEC_ALTERNATIVE
> > but only use them for "objtool orc generate".
> > 
> 
> I have checked and objtool doesn't emit ORC entries for alternative:
> decode_instructions() doesn't mark such section with sec->text = true
> so create_orc_sections() doesn't emit corresponding ORC entries.
> 
> So I think we can remove the ANNOTATE_NOSPEC_ALTERNATIVE directives,
> this will allow objtool to check the instructions but it still won't
> emit ORC entries (same behavior as today). In the future, if ORC
> eventually supports alternative we will be ready to have objtool emit
> ORC entries.

What's the benefit of removing ANNOTATE_NOSPEC_ALTERNATIVE if there's no
ORC support to go along with it?

Also I want to avoid adding "ORC alternatives".  ORC is nice and simple
and we should keep it that way as much as possible.

Again, we should warn on stack changes inside alternatives, and then
look at converting RSB and retpolines to use static branches so they
have deterministic stacks.

-- 
Josh