linux-kernel - Re: [RFC PATCH 3/3] objtool/mcount: Add powerpc specific functions

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Message-Id: <1647843607.5vkv4b5tvk.naveen@linux.ibm.com>
Date:   Mon, 21 Mar 2022 11:55:41 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Sathvika Vasireddy <sv@...ux.ibm.com>
Cc:     aik@...abs.ru, jpoimboe@...hat.com, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, mpe@...erman.id.au,
        rostedt@...dmis.org
Subject: Re: [RFC PATCH 3/3] objtool/mcount: Add powerpc specific functions

Peter Zijlstra wrote:
> On Fri, Mar 18, 2022 at 04:21:40PM +0530, Sathvika Vasireddy wrote:
>> This patch adds powerpc specific functions required for
>> 'objtool mcount' to work, and enables mcount for ppc.
> 
> I would love to see more objtool enablement for Power :-)
> 
> 
>> diff --git a/tools/objtool/arch/powerpc/include/arch/elf.h b/tools/objtool/arch/powerpc/include/arch/elf.h
>> new file mode 100644
>> index 000000000000..3c8ebb7d2a6b
>> --- /dev/null
>> +++ b/tools/objtool/arch/powerpc/include/arch/elf.h
>> @@ -0,0 +1,8 @@
>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>> +
>> +#ifndef _OBJTOOL_ARCH_ELF
>> +#define _OBJTOOL_ARCH_ELF
>> +
>> +#define R_NONE R_PPC_NONE
>> +
>> +#endif /* _OBJTOOL_ARCH_ELF */
>> diff --git a/tools/objtool/utils.c b/tools/objtool/utils.c
>> index d1fc6a123a6e..c9c14fa0dfd7 100644
>> --- a/tools/objtool/utils.c
>> +++ b/tools/objtool/utils.c
>> @@ -179,11 +179,29 @@ int create_mcount_loc_sections(struct objtool_file *file)
>>  		loc = (unsigned long *)sec->data->d_buf + idx;
>>  		memset(loc, 0, sizeof(unsigned long));
>>  
>> -		if (elf_add_reloc_to_insn(file->elf, sec,
>> -					  idx * sizeof(unsigned long),
>> -					  R_X86_64_64,
>> -					  insn->sec, insn->offset))
>> -			return -1;
>> +		if (file->elf->ehdr.e_machine == EM_X86_64) {
>> +			if (elf_add_reloc_to_insn(file->elf, sec,
>> +						  idx * sizeof(unsigned long),
>> +						  R_X86_64_64,
>> +						  insn->sec, insn->offset))
>> +				return -1;
>> +		}
>> +
>> +		if (file->elf->ehdr.e_machine == EM_PPC64) {
>> +			if (elf_add_reloc_to_insn(file->elf, sec,
>> +						  idx * sizeof(unsigned long),
>> +						  R_PPC64_ADDR64,
>> +						  insn->sec, insn->offset))
>> +				return -1;
>> +		}
>> +
>> +		if (file->elf->ehdr.e_machine == EM_PPC) {
>> +			if (elf_add_reloc_to_insn(file->elf, sec,
>> +						  idx * sizeof(unsigned long),
>> +						  R_PPC_ADDR32,
>> +						  insn->sec, insn->offset))
>> +				return -1;
>> +		}
> 
> It appears to me that repeating this code 3 times doesn't really scale
> well, how about we introduce a helper like:
> 
> 
> int elf_reloc_type_long(struct elf *elf)
> {
> 	switch (elf->ehdr.e_machine) {
> 	case EM_X86_64:
> 		return R_X86_64_64;
> 	case EM_PPC64:
> 		return R_PPC64_ADDR64;
> 	case EM_PPC:
> 		return R_PPC_ADDR32;
> 	default:
> 		WARN("unknown machine...")
> 		exit(-1);
> 	}
> }
> 
> 		if (elf_add_reloc_to_insn(file->elf, sec,
> 					  idx * sizeof(unsigned long),
> 					  elf_reloc_type_long(file->elf),
> 					  insn->sec, insn->offset))
> 			return -1;

Good point. I suppose we'll need a similar helper, say, 
elf_reloc_type_none(), to replace R_NONE usage if we want to have 
objtool work for cross-arch builds.


- Naveen