Re: [PATCH] Adding Bi-Endian 32-bit and 64-bit Support to the Grub ELF Parser

[Colin Watson <cjwatson@ubuntu.com>]

On Fri, Jan 10, 2014 at 11:52:52PM +0400, Andrey Borzenkov wrote:
> В Fri, 10 Jan 2014 11:58:58 -0600
> Tomohiro B Berry <tbberry@us.ibm.com> пишет:
> > This patch adds bi-endian support for both 32-bit and 64-bit elf files. 
> > 
> > It compares the native endianness to the endianness of the elf file, and 
> > swaps the header bytes if necessary.  This will allow, for example, 32-bit 
> > Big Endian grub to load a 64-bit Little Endian kernel. 
> 
> I wonder - when big endian grub will pass boot parameters to little
> endian kernel - won't there be endianness mismatch?

I think this is probably not a problem for the case at hand (powerpc),
because command-line arguments are passed by way of an IEEE1275 property
rather than using a pointer in a structure as we do on x86.  And the
kernel expects to enter the PROM in 32-bit BE mode, so passing its
address that way round should be fine.

However, Tomohiro, I wonder if you have tested this with an initrd?  I
don't see code in the kernel to cope with a byteswapped initrd address
and size (though I certainly could be missing something).

If that side of things is confirmed to work, then this looks mostly fine
to me.

I wonder if perhaps we ought to only do this on architectures where we
know that the kernel can cope with being entered in the wrong
endianness?  I don't know - maybe we don't need to worry as on other
architectures the chances of running across a wrong-endian kernel are
pretty infinitesimal anyway.

-- 
Colin Watson                                       [cjwatson@ubuntu.com]