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Re: [Qemu-devel] [PATCH] Fix -kernel on target-ppc

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH] Fix -kernel on target-ppc
Date: Sun, 25 Jan 2009 11:24:38 +0100

On 25.01.2009, at 11:16, Edgar E. Iglesias wrote:

On Sun, Jan 25, 2009 at 11:03:47AM +0100, Alexander Graf wrote:

On 25.01.2009, at 10:54, Aurelien Jarno wrote:

On Sun, Jan 25, 2009 at 01:28:36AM +0100, Alexander Graf wrote:

On 25.01.2009, at 00:59, Aurelien Jarno <address@hidden> wrote:

On Sat, Jan 24, 2009 at 10:57:19PM +0100, Alexander Graf wrote:

On 24.01.2009, at 22:48, Aurelien Jarno wrote:

On Sat, Jan 24, 2009 at 09:58:35PM +0100, Alexander Graf wrote:
OpenBIOS searches for the preloaded kernel at 0x1000000, so let's
put it there instead of an invalid location.

Your patch is actually wrong, the second argument of load_elf() is
offset to the physical address (as found in the elf header), and
not a
load address.

By chance the physical address of a >= 2.6.25 kernel is
0x00000000, so
your patch works. But it will break supports for < 2.6.25 kernel as
their physical address is 0xc0000000. Not that they are only the
values, they can be changed in the .config file.

Aah, that explains why :-).

I have already proposed a patch to use the virtual address of the
header as done by yaboot or quik, but I have been told it is

We have to find another way to load the elf file at a fixed address.

Hm - can't we just give load_elf an override value for the base

         /* address_offset is hack for kernel images that are
            linked at the wrong physical address.  */
         addr = ph->p_paddr + address_offset;

         cpu_physical_memory_write_rom(addr, data, mem_size);

Just pass another variable here that overrides addr and doesn't
calculate it?

Except that they can be more than one segment to load, so the last one will overwrite the previous ones. The PowerPC kernels I have seen only
have one load segment, but I am not sure it is always the case.

But then the addr hack wouldn't work either, right? It's just a question
if addr_offset is relative or absolute here.

addr_offset is just an offset that is added to the load address of the
elf header.

And fwiw in this case relative to the elf header's value doesn't make
any sense at all when the firmware expects the blob on a specific

As far as I understand it has been done like that to be able to support multiple segments. If the elf header says that the first segment has to be loaded at 0xc0000000 and the second at 0xd0000000, loading both at 0x10000000 won't work. Loading them with an offset of -0xb000000 will
load the first one at 0x10000000 and the second one just after at

Aaah I'm starting to see the picture now :-). Sorry, I probably shouldn't
do this on a weekend at night.

So at least for the Linux kernel case it would be enough to know which lowest address the kernel is loaded to, right? Because that's what we need
to use as negative offset.

So if we call load_elf twice, once to get the lowest address the binary is loaded to (lowaddr) and the second one to actually load it, this should at least fix it for Linux without breaking multiple segment support, as long
as the first segment is the text segment.

Am I getting this right?

I think so, that same kind of speculative loading is what I had in mind:

Ah :-). Does anyone have an old and a Haiku kernel lying around he could point me to so I can write a small patch?

Also has anyone gotten console output from loaded kernels? For me the kernel switches consoles from udbg0 to .. eh ... something. And after that nothing gets displayed anymore. -append console=ttyS0 doesn't work either.

Btw - is cdrom booting supported yet?


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