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

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Message-ID: <4e5572cc-a134-0a66-9b09-e0aa7ff5102a@oracle.com>
Date:   Tue, 7 Apr 2020 19:04:28 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, 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 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?
>