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

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Date:   Thu, 16 Jun 2022 19:04:20 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        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

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?


- Naveen