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

From: Alexandre Chartre
Date: Tue Apr 07 2020 - 13:00:21 EST



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?