How does loader(8) decide where to load the kernel?

[John Baldwin]

On Saturday, May 05, 2012 1:06:13 am Tim Kientzle wrote:
> I have ubldr loading the ELF kernel on BeagleBone and am now
> trying to untangle some of the hacks I used to get this working.
> 
> Unfortunately, there's one area of the common loader(8) code
> that I really don't understand:  How does sys/boot/common/load_elf.c
> determine the physical address at which to load the kernel?
> 
> __elfN(loadfile) has the following comment:
> 
>      [The file] will be stored at (dest).
> 
> But that's not really true.  For starters, loadfile maps dest
> through archsw.arch_loadaddr.   (This mechanism seems
> to only be used on ia64 and pc98, though the result is
> later discarded on those platforms.)
> 
> Loadfile then passes the value to loadimage which does
> very strange things:
> 
> On i386, amd64, powerpc, and arm,  loadimage subtracts
> the dest value from the address declared in the actual ELF
> headers so that the kernel always gets loaded into low memory.
> (there's some intermediate bit-twiddling I'm glossing over, but
> this is the general idea).

The bit twiddling is supposed to be the equivalent of subtracting
KERNBASE from the load address.  On both i386 and amd64, there is
a direct mapping of the kernel text such that KERNBASE maps address
0, etc.  By default on i386 KERNBASE is 0xc0000000.

-- 
John Baldwin