[Qemu-devel] [PATCH][RFC] elf loader: use the virtual address

[Qemu-devel] [PATCH][RFC] elf loader: use the virtual address

[Aurelien Jarno]

Most Linux kernels have their physical address corresponding to they
virtual address in the ELF header. This is however not true anymore for
the PowerPC kernels (>= 2.6.25).

For PowerPC, the kernel needs to be started with address translation
enabled (that's even true for the firmware), and thus the kernel loaded
at the virtual address.

As all the other kernels/bios I have looked have the same virtual and
physical address, I don't think it will break other targets. This is
what is done in the patch below. Alternatively, we can add a new 
argument to the load_elf functions, to select between virtual and
physical load address.

Any opinon?

diff --git a/elf_ops.h b/elf_ops.h
index feea12f..19d1a5e 100644
--- a/elf_ops.h
+++ b/elf_ops.h
@@ -231,7 +231,7 @@ static int glue(load_elf, SZ)(int fd, int64_t address_offset,
             }
             /* address_offset is hack for kernel images that are
                linked at the wrong physical address.  */
-            addr = ph->p_paddr + address_offset;
+            addr = ph->p_vaddr + address_offset;
 
             cpu_physical_memory_write_rom(addr, data, mem_size);

-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32@debian.org         | aurelien@aurel32.net
   `-    people.debian.org/~aurel32 | www.aurel32.net

Re: [Qemu-devel] [PATCH][RFC] elf loader: use the virtual address

[Laurent Desnogues]

On Fri, Jan 9, 2009 at 4:47 PM, Aurelien Jarno <aurelien@aurel32.net> wrote:
> Most Linux kernels have their physical address corresponding to they
> virtual address in the ELF header. This is however not true anymore for
> the PowerPC kernels (>= 2.6.25).
>
> For PowerPC, the kernel needs to be started with address translation
> enabled (that's even true for the firmware), and thus the kernel loaded
> at the virtual address.
>
> As all the other kernels/bios I have looked have the same virtual and
> physical address, I don't think it will break other targets. This is
> what is done in the patch below. Alternatively, we can add a new
> argument to the load_elf functions, to select between virtual and
> physical load address.
>
> Any opinon?

I wonder if some platforms don't use the address_offset parameter
to simulate that behaviour.

Anyway wouldn't a parameter be cleaner and allow more flexibility?


Laurent

> diff --git a/elf_ops.h b/elf_ops.h
> index feea12f..19d1a5e 100644
> --- a/elf_ops.h
> +++ b/elf_ops.h
> @@ -231,7 +231,7 @@ static int glue(load_elf, SZ)(int fd, int64_t address_offset,
>             }
>             /* address_offset is hack for kernel images that are
>                linked at the wrong physical address.  */
> -            addr = ph->p_paddr + address_offset;
> +            addr = ph->p_vaddr + address_offset;
>
>             cpu_physical_memory_write_rom(addr, data, mem_size);
>
> --
>  .''`.  Aurelien Jarno             | GPG: 1024D/F1BCDB73
>  : :' :  Debian developer           | Electrical Engineer
>  `. `'   aurel32@debian.org         | aurelien@aurel32.net
>   `-    people.debian.org/~aurel32 | www.aurel32.net
>
>
>

Re: [Qemu-devel] [PATCH][RFC] elf loader: use the virtual address

[Aurelien Jarno]

Laurent Desnogues a écrit :
> On Fri, Jan 9, 2009 at 4:47 PM, Aurelien Jarno <aurelien@aurel32.net> wrote:
>> Most Linux kernels have their physical address corresponding to they
>> virtual address in the ELF header. This is however not true anymore for
>> the PowerPC kernels (>= 2.6.25).
>>
>> For PowerPC, the kernel needs to be started with address translation
>> enabled (that's even true for the firmware), and thus the kernel loaded
>> at the virtual address.
>>
>> As all the other kernels/bios I have looked have the same virtual and
>> physical address, I don't think it will break other targets. This is
>> what is done in the patch below. Alternatively, we can add a new
>> argument to the load_elf functions, to select between virtual and
>> physical load address.
>>
>> Any opinon?
> 
> I wonder if some platforms don't use the address_offset parameter
> to simulate that behaviour.

PowerPC already does that. The offset is relative to the load address,
so it should still be read correctly.

> Anyway wouldn't a parameter be cleaner and allow more flexibility?
> 

Maybe that's what I am going to implement.

-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32@debian.org         | aurelien@aurel32.net
   `-    people.debian.org/~aurel32 | www.aurel32.net

Re: [Qemu-devel] [PATCH][RFC] elf loader: use the virtual address

[Paul Brook]

> For PowerPC, the kernel needs to be started with address translation
> enabled (that's even true for the firmware), and thus the kernel loaded
> at the virtual address.

This sounds wrong.

Surely we should be loading it at the paddr with translation disabled, hat's 
the whole point of having both paddr and vaddr in the ELF headers. If the ppc 
kernel isn't setting p_paddr to the load address then what is it setting it 
to?

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

> As all the other kernels/bios I have looked have the same virtual and
> physical address, I don't think it will break other targets.

This will definitely break things. In particular bare metal applications built 
for the Luminary Micro boards (and probably any other system that uses XIP 
from flash).  I also have several non-linux OS and bootloaders that rely on 
the current elf loader behavior.
It's standard practice to use the paddr when loading an image. This si 
consistent with gdb and several boot loaders.

See also http://lists.gnu.org/archive/html/qemu-devel/2008-10/msg00864.html

Paul

Re: [Qemu-devel] [PATCH][RFC] elf loader: use the virtual address

[Edgar E. Iglesias]

On Fri, Jan 09, 2009 at 03:38:58PM -0800, Paul Brook wrote:
> > For PowerPC, the kernel needs to be started with address translation
> > enabled (that's even true for the firmware), and thus the kernel loaded
> > at the virtual address.
> 
> This sounds wrong.

I agree. It sounds to me like if powerpc linux changed their linking
to produce elf files with correct physical addresses and that makes it hard
for qemu to support both the new and the old images.

Maybe ppc could use the low/highaddr arguments to find out if the kernel
objects really needs to use hte -0xc00000000 offset or no?

my guess is that older kernels need that offset but that the newer ones
dont.

Re: [Qemu-devel] [PATCH][RFC] elf loader: use the virtual address

[Aurelien Jarno]

On Fri, Jan 09, 2009 at 03:38:58PM -0800, Paul Brook wrote:
> > For PowerPC, the kernel needs to be started with address translation
> > enabled (that's even true for the firmware), and thus the kernel loaded
> > at the virtual address.
> 
> This sounds wrong.
> 
> Surely we should be loading it at the paddr with translation disabled, hat's 
> the whole point of having both paddr and vaddr in the ELF headers. If the ppc 
> kernel isn't setting p_paddr to the load address then what is it setting it 
> to?

It was set to the same value as the virtual address (0xc0000000), and it
is now set to 0x00000000.

> >              /* address_offset is hack for kernel images that are
> >                 linked at the wrong physical address.  */
> > -            addr = ph->p_paddr + address_offset;
> > +            addr = ph->p_vaddr + address_offset;
> 
> > As all the other kernels/bios I have looked have the same virtual and
> > physical address, I don't think it will break other targets.
> 
> This will definitely break things. In particular bare metal applications built 
> for the Luminary Micro boards (and probably any other system that uses XIP 
> from flash).  I also have several non-linux OS and bootloaders that rely on 
> the current elf loader behavior.
> It's standard practice to use the paddr when loading an image. This si 
> consistent with gdb and several boot loaders.
> 

If we want to mimic bootloaders, both quik and yaboot are using
p_vaddr.

-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32@debian.org         | aurelien@aurel32.net
   `-    people.debian.org/~aurel32 | www.aurel32.net