On Apr 3, 2008, at 1:34 AM, Paul Mackerras wrote:
Kumar Gala writes:
For a kdump kernel, at least for 64-bit, the physical address has to
be 32MB. There is no other choice, so there is no possibility of
confusion.
But how do you know a vmlinux image is for kdump or not?
By looking at the ELF headers -- either the first PT_LOAD segment or
the .text section -- and seeing whether the start address is
0xc000000002000000 or not.
fair point.
For 85xx, would it be possible to have the kernel figure out what
physical address it has been loaded at, and use that as the base
address, rather than having the base address set at compile time?
Yes, that is what CONFIG_RELOCATABLE is all about.
Is there any reason to have that as an option, rather than just always
doing that?
Concerned about runtime performance of __va() and __pa().
That would solve my objection since it would mean that there would
no
longer be two things that had to be kept in sync. You could pass
any
value to wrapper/mkimage (subject to constraints such as it has to
be
a multiple of 256M) and it would work. That value could even come
from a config option in the case where wrapper is invoked as part of
the kernel build, but that config option shouldn't affect anything
at
all in the vmlinux.
Ok, but I still think the issues exists when we config PHYSICAL_START
to non-zero and CONFIG_RELOCATABLE=n. Ideally we get set the phys
address the PHDR, but I'm not sure how to get the linker to do that.
If we can do that then the wrapper script can dig it out and pass it
to mkimage, which would solve the problem.
Ok, so on Segher's recommendation I looked at 'AT' and posted a patch
that uses that so now regardless of the kernel the PT_LOAD PHDR will
have the physical address set properly.
So now we can look at the vmlinux and determine the physical offset.
The question is how best to do that. Here are the options I see:
* readelf, grep and parse output
* objdump grep and parse output
* simple C program that read's the elf and reports back
[looking for suggestion on what direction you want to take]
The other questions is if we'd ever have a vmlinux with more than one
PT_LOAD PHDR. If so which one do we use (the one with the lowest
physical address)?
- k
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