linux-kernel - Re: [PATCH v3 4/4] objtool: orc_gen: Move orc

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Date:   Thu, 30 Jul 2020 14:45:46 +0100
From:   Julien Thierry <jthierry@...hat.com>
To:     peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        mhelsley@...are.com, mbenes@...e.cz
Subject: Re: [PATCH v3 4/4] objtool: orc_gen: Move orc_entry out of
 instruction structure



On 7/30/20 2:33 PM, peterz@...radead.org wrote:
> On Thu, Jul 30, 2020 at 01:40:48PM +0100, Julien Thierry wrote:
>>
>>
>> On 7/30/20 11:03 AM, peterz@...radead.org wrote:
>>> On Thu, Jul 30, 2020 at 10:41:43AM +0100, Julien Thierry wrote:
>>>> One orc_entry is associated with each instruction in the object file,
>>>> but having the orc_entry contained by the instruction structure forces
>>>> architectures not implementing the orc subcommands to provide a dummy
>>>> definition of the orc_entry.
> 
>> I guess I forgot about the usecase of running objtool on vmlinux...
> 
> Right, and LTO builds will even do ORC at that level.
> 
>> On a kernel build for x86_64 defconfig, the difference in time seems to be
>> withing the noise.
> 
> Good.
> 
>> But I agree the proposed code is not ideal and on the other we've tried
>> avoiding #ifdef in the code. Ideally I'd have an empty orc_entry definition
>> when SUBCMD_ORC is not implemented.
>>
>> Would you have a suggested approach to do that?
> 
> How ugly is having that:
> 
> struct orc_entry { };
> 
> ?

Not sure I am understanding the suggestion. Without #ifdef this will 
conflict with the definition in <asm/orc_types.h> for x86. Or every arch 
needs to provide their own <asm/orc_types.h> and definition of struct 
orc_entry, even if they don't implement the orc subcommand.

Which would be preferable? #ifdef? or arch provided definition? (or 
something I have not thought of)

-- 
Julien Thierry