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

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Date:   Wed, 25 May 2022 19:27:31 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Sathvika Vasireddy <sv@...ux.vnet.ibm.com>
Cc:     "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "aik@...abs.ru" <aik@...abs.ru>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "naveen.n.rao@...ux.vnet.ibm.com" <naveen.n.rao@...ux.vnet.ibm.com>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RFC PATCH 4/4] objtool/powerpc: Add --mcount specific
 implementation



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)

Christophe