Re: [PATCH 1/3] x86: record relocation offset

H. Peter Anvin
2009-12-30T22:14:47Z

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Are we concerned about virtual or physical addresses, here? I'm assuming virtual; in that case do note that we only actually relocate the kernel on 32 bits - on 64 bits the relocation is done at the page table level since we need the high map anyway.

On 32 bits one can compare any one symbol before and after relocation - it obviously doesn't matter which symbol as long as it is the same. The kernel start will be given by _text or startup_32; if that feels too "fuzzy" we could of course add a specific kernel start symbol explicitly for that purpose.

"Arnaldo Carvalho de Melo" <acme@infradead.org> wrote:

>Em Wed, Dec 30, 2009 at 06:39:36PM -0200, Arnaldo Carvalho de Melo escreveu:
>> Em Wed, Dec 30, 2009 at 11:45:30AM -0800, H. Peter Anvin escreveu:
>> > The kernel already knows where it is loaded -- obviously, by sheer
>> > necessity -- and knows how it was itself configured, and as such we
>> > can do this calculation in C code without modifying boot_params or
>> > the early bootstrap.
>>
>> Problem is that at 'perf record' time we may not have access to the
>> vmlinux file, and thus not be able to figure out the relocation applied
>> in that boot.
>>
>> Then, at a later time, and possibly on another machine, on another arch,
>> we try to map back IPs to symbols, the /proc/kallsyms is completely
>> unrelated and we now have a vmlinux unrelocated...
>>
>> So we need a way to get the relocation applied at 'perf record' time and
>> encode it in the perf.data header. Ideas about how to do that?
>
>Well, I guess we could do the _stext trick again, storing its value,
>taken from /proc/kallsyms, into the perf.data header, then figuring out
>the relocation by looking at its value in the vmlinux symtab.
>
>There were concerns in the past about relying on _stext, IIRC, James?
>
>- Arnaldo

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