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

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Date:   Tue, 7 Apr 2020 18:18:51 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 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.

alex.