linux-kernel - Re: [RFC PATCH 4/4] objtool/powerpc: Add --mcount specific implementation

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Message-ID: <30f3791c-0fdd-e635-4a85-ec457f990fae@csgroup.eu>
Date:   Thu, 16 Jun 2022 13:40:34 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Sathvika Vasireddy <sv@...ux.vnet.ibm.com>
CC:     "aik@...abs.ru" <aik@...abs.ru>,
        Chen Zhongjin <chenzhongjin@...wei.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>
Subject: Re: [RFC PATCH 4/4] objtool/powerpc: Add --mcount specific
 implementation



Le 16/06/2022 à 15:34, Naveen N. Rao a écrit :
> Christophe Leroy wrote:
>>
>>
>> Le 25/05/2022 à 19:27, Christophe Leroy a écrit :
>>>
>>>
>>> Le 24/05/2022 à 15:33, Christophe Leroy a écrit :
>>>>
>>>>
>>>> Le 24/05/2022 à 13:00, Sathvika Vasireddy a écrit :
>>>>>>
>>>>>>> +{
>>>>>>> +    switch (elf->ehdr.e_machine) {
>>>>>>> +    case EM_X86_64:
>>>>>>> +            return R_X86_64_64;
>>>>>>> +    case EM_PPC64:
>>>>>>> +            return R_PPC64_ADDR64;
>>>>>>> +    default:
>>>>>>> +            WARN("unknown machine...");
>>>>>>> +            exit(-1);
>>>>>>> +    }
>>>>>>> +}
>>>>>> Wouldn't it be better to make that function arch specific ?
>>>>>
>>>>> This is so that we can support cross architecture builds.
>>>>>
>>>>
>>>>
>>>> I'm not sure I follow you here.
>>>>
>>>> This is only based on the target, it doesn't depend on the build 
>>>> host so
>>>> I can't the link with cross arch builds.
>>>>
>>>> The same as you have arch_decode_instruction(), you could have
>>>> arch_elf_reloc_type_long()
>>>> It would make sense indeed, because there is no point in supporting X86
>>>> relocation when you don't support X86 instruction decoding.
>>>>
>>>
>>> Could simply be some macro defined in 
>>> tools/objtool/arch/powerpc/include/arch/elf.h and 
>>> tools/objtool/arch/x86/include/arch/elf.h
>>>
>>> The x86 version would be:
>>>
>>> #define R_ADDR(elf) R_X86_64_64
>>>
>>> And the powerpc version would be:
>>>
>>> #define R_ADDR(elf) (elf->ehdr.e_machine == EM_PPC64 ? R_PPC64_ADDR64 
>>> : R_PPC_ADDR32)
>>>
>>
>> Well, looking once more, and taking into account the patch from Chen 
>> https://lore.kernel.org/lkml/20220531020744.236970-4-chenzhongjin@huawei.com/ 
>>
>>
>> It would be easier to just define two macros:
>>
>> #define R_ABS64 R_PPC64_ADDR64
>> #define R_ABS32 R_PPC_ADDR32
>>
>> And then in the caller, as we know the size, do something like
>>
>>     size == sizeof(u64) ? R_ABS64 : R_ABS32;
> 
> How does 'sizeof(u64)' work here?
> 

sizeof(u64) is always 8 by definition.

So if size is 8 we are working on a binary file for a 64 bits target, if 
not it means we are working for a 32 bits target.

Christophe