[OpenBIOS] PPC: QEMU breaks static variables in OpenBIOS

[Mark Cave-Ayland]

On 02/01/13 21:07, Benjamin Herrenschmidt wrote:

> Just relocate .data, that's trivial.
>
> Use the .lds to specify a different load address from link address, put
> the link address in memory along with the bss and in your early startup
> code, copy them from the load address to the link address.
>
> No need to process ELF relocs or anything like that.
>
> I'm sure you can find examples of how to do that all over the place
> (uboot would be a place to look if you don't know the .lds syntax for
> this, off the top of my head you look for the AT() directive).
>
> Cheers,
> Ben.

Hi Ben,

Thanks for the response. The existing comments in arch/ppc/qemu/start.S 
suggest that we have a 1MB space at the top of (physical) RAM which we 
could use for this:

         /* Memory map:
          *
          * Top +-------------------------+
          *     |                         |
          *     | ROM into RAM (1 MB)     |
          *     |                         |
          *     +-------------------------+
          *     |                         |
          *     | MMU Hash Table (64 kB)  |
          *     |                         |
          *     +-------------------------+
          *     |                         |
          *     | Exception Stack (32 kB) |
          *     |                         |
          *     +-------------------------+
          *     |                         |
          *     | Stack (64 kB)           |
          *     |                         |
          *     +-------------------------+
          *     |                         |
          *     | Client Stack (64 kB)    |
          *     |                         |
          *     +-------------------------+
          *     |                         |
          *     | Malloc Zone (2 MiB)     |
          *     |                         |
          *     +-------------------------+
          *     :                         :
          * Bottom
          */

The comments in arch/ppc/ofmem.c suggest that virtual memory looks like 
this:

/****************************************************************
  * Memory usage (before of_quiesce is called)
  *
  *                      Physical
  *
  *      0x00000000      Exception vectors
  *      0x00004000      Free space
  *      0x01e00000      Open Firmware (us)
  *      0x01f00000      OF allocations
  *      0x01ff0000      PTE Hash
  *      0x02000000-     Free space
  *
  * Allocations grow downwards from 0x01e00000
  *
  ****************************************************************/

So could we stash .data somewhere beneath the Open Firmware virtual 
mapping, e.g. at 0x01d00000?

Ben: is this a suitable location for the data section? AFAICT the Open 
Firmware load base is set to 0x4000 so we shouldn't in theory clash with 
any boot loaders/kernels. (Disclaimer: I've mainly worked on the SPARC 
side of OpenBIOS to date, so a lot of the PPC stuff is new to me)

Alex/Andreas: is this something that you would be able to help with as 
PPC maintainers? In particular, I have no way to test PPC64. I could 
probably make an attempt at something for PPC32, with the minor caveat 
that I would have to learn PPC assembler first...


ATB,

Mark.