Re: [PATCH V2 9/9] x86/speculation: Remove all ANNOTATE_NOSPEC_ALTERNATIVE directives

[Alexandre Chartre]

On 4/7/20 6:41 PM, Peter Zijlstra wrote:
> 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.
>
> I mean, we should make it warn for the case where you remove
> ANNOTATE_NOSPEC and it would like to generate ORC.

Okay, so check if an alternative is changing the stack and warn if it is.

alex.

> Also, what's the point of having objtool grok this code and then not
> doing anything with it?