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

[Alexander Graf]

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
> > > > > an
> > > > > 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
> > > > > default
> > > > > 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
> > > > > elf
> > > > > header as done by yaboot or quik, but I have been told it is
> > > > > actually
> > > > > wrong.
> > > > >
> > > > > 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?
> > > >
> > > >           /* 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
> > address.
>
> 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
> 0x20000000.

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?

Alex