x86: Executing a raw vmlinux image (embedded environment)

x86: Executing a raw vmlinux image (embedded environment)

[Graeme Russ]

Hi All,

Firstly, a little introduction an background - I am currently the maintainer
for the x86 port of Das U-Boot (or more simply U-Boot). While the x86 port
is not as well known as the more major ports like ARM and PPC, it is
starting to gain more attention and more developer input which has beed
nice to see after my solo efforts over the last few years.

One of my biggest annoyances with the x86 U-Boot port is that, while it is
an embedded boot loader, it still tries to boot Linux as if it was a
conventional PC - i.e. it has 'real mode' and 'BIOS' implementation and
loads a bzImage. But this approach is completely unnecessary and only
adds to the boot time (load bzImage from storage into RAM, decompress,
then run).

So I want to shortcut bzImage and migrate towards treating the loading
of the Linux kernel from U-Boot like any other embedded environment:
 - Decompress vmlinux directly from storage into the appropriate memory
   location
 - Setup required data structures
 - Jump into vmlinux

U-Boot has it's own compressed kernel image container (uImage) and support
code which will allow me to decompress the vmlinux directly from storage
into RAM. In my current situation, I have ~1.7MB of onboard (i.e. cached)
flash memory where I plan to store the compressed kernel and an MMC where
I plan to store the file system.

So after building a very stripped down kernel (no TCP/IP for example) I
get:
       /vmlinux                                        2,629,659  bytes
       /vmlinux.o                                      2,889,050  bytes
       /arch/i386/boot/bzImage                         1,104,864  bytes
       /arch/x86/boot/bzImage                          1,104,864  bytes
       /arch/x86/boot/vmlinux.bin                      1,092,060  bytes
       /arch/x86/boot/compressed/vmlinux               1,099,538  bytes
       /arch/x86/boot/compressed/vmlinux.bin           2,094,132  bytes
       /arch/x86/boot/compressed/vmlinux.bin.gz        1,074,711  bytes

I understand that /arch/x86/boot/compressed/vmlinux.bin.gz is a compressed
version of /arch/x86/boot/compressed/vmlinux.bin, and
/arch/i386/boot/bzImage and /arch/x86/boot/bzImage are the same file and
that it is the 16-bit boot code + /arch/x86/boot/compressed/vmlinux.bin.gz
but I don't understand the rest...

My guess is that /vmlinux.o is the ELF image generated by the compiler +
linker stage and /vmlinux may be /vmlinux.o objdump'd into a raw binary and
perhaps /arch/x86/boot/vmlinux.bin is a further stripped version
of/vmlinux, but I'm at a loss with /arch/x86/boot/compressed/vmlinux

In any event, it looks like either /arch/x86/boot/compressed/vmlinux.bin or
/vmlinux is what I need to copy into RAM @ 0x100000 (1MiB) which is where
my non-relocatable kernel is compiled to.

I also have looked at the documentation for the x86 32-boot protocol found
in linux/Documentation/x86/boot.txt

So what I'm needing is:
 - Confirmation of exactly which vmlinux to use
 - Confirmation that I do load it @ 0x100000
 - How to setup the memory map (keeping in mind I have 2GB of contiguous
   memory with no BIOS/ACPI etc to worry about clobbering
 - Any other tricks I need to be aware of..

Any help will be greatly appreciated

Thanks,

Graeme

x86: Executing a raw vmlinux image (embedded environment)

[sk.syed2]

> ? ? ? /vmlinux ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?2,629,659 ?bytes
> ? ? ? /vmlinux.o ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?2,889,050 ?bytes
> ? ? ? /arch/i386/boot/bzImage ? ? ? ? ? ? ? ? ? ? ? ? 1,104,864 ?bytes
> ? ? ? /arch/x86/boot/bzImage ? ? ? ? ? ? ? ? ? ? ? ? ?1,104,864 ?bytes
> ? ? ? /arch/x86/boot/vmlinux.bin ? ? ? ? ? ? ? ? ? ? ?1,092,060 ?bytes
> ? ? ? /arch/x86/boot/compressed/vmlinux ? ? ? ? ? ? ? 1,099,538 ?bytes
> ? ? ? /arch/x86/boot/compressed/vmlinux.bin ? ? ? ? ? 2,094,132 ?bytes
> ? ? ? /arch/x86/boot/compressed/vmlinux.bin.gz ? ? ? ?1,074,711 ?bytes
>
> I understand that /arch/x86/boot/compressed/vmlinux.bin.gz is a compressed
> version of /arch/x86/boot/compressed/vmlinux.bin, and
> /arch/i386/boot/bzImage and /arch/x86/boot/bzImage are the same file and
> that it is the 16-bit boot code + /arch/x86/boot/compressed/vmlinux.bin.gz
This is correct.
> but I don't understand the rest...

>
> My guess is that /vmlinux.o is the ELF image generated by the compiler +
> linker stage and /vmlinux may be /vmlinux.o objdump'd into a raw binary and
> perhaps /arch/x86/boot/vmlinux.bin is a further stripped version
> of/vmlinux, but I'm at a loss with /arch/x86/boot/compressed/vmlinux

vmlinux is ELF image with ELF header. So actual point of kernel entry
would be at an offset, somewhere after the ELF header.
vmlinux.bin is what you would get after doing
#objcopy -O binary vmlinux vmlinux.bin.
vmlinux.bin has only obj code and nothing else.
>
> In any event, it looks like either /arch/x86/boot/compressed/vmlinux.bin or
> /vmlinux is what I need to copy into RAM @ 0x100000 (1MiB) which is where
> my non-relocatable kernel is compiled to.

copy vmlinux.bin.

> ?- How to setup the memory map (keeping in mind I have 2GB of contiguous
> ? memory with no BIOS/ACPI etc to worry about clobbering
> ?- Any other tricks I need to be aware of..
Check if x86 kernel expects some parameters(like machineid, bootargs
location etc) in some registers.
Check if x86 has low level debug support(like DEBUG_LL).
Also you might want to check how initial page tables are being setup in kernel.

-syed

x86: Executing a raw vmlinux image (embedded environment)

[Graeme Russ]

Thanks syed,

Ok, I did a little more digging...

On 22/11/11 16:34, sk.syed2 wrote:
>>       /vmlinux                                        2,629,659  bytes
>>       /vmlinux.o                                      2,889,050  bytes
>>       /arch/i386/boot/bzImage                         1,104,864  bytes
>>       /arch/x86/boot/bzImage                          1,104,864  bytes
>>       /arch/x86/boot/vmlinux.bin                      1,092,060  bytes
>>       /arch/x86/boot/compressed/vmlinux               1,099,538  bytes
>>       /arch/x86/boot/compressed/vmlinux.bin           2,094,132  bytes
>>       /arch/x86/boot/compressed/vmlinux.bin.gz        1,074,711  bytes
>>
>> I understand that /arch/x86/boot/compressed/vmlinux.bin.gz is a compressed
>> version of /arch/x86/boot/compressed/vmlinux.bin, and
>> /arch/i386/boot/bzImage and /arch/x86/boot/bzImage are the same file and
>> that it is the 16-bit boot code + /arch/x86/boot/compressed/vmlinux.bin.gz
> This is correct.
>> but I don't understand the rest...

I traced it all out. I sent a separate message to the ML documenting the
bzImage build chain - It's rather fascinating

> 
>>
>> My guess is that /vmlinux.o is the ELF image generated by the compiler +
>> linker stage and /vmlinux may be /vmlinux.o objdump'd into a raw binary and
>> perhaps /arch/x86/boot/vmlinux.bin is a further stripped version
>> of/vmlinux, but I'm at a loss with /arch/x86/boot/compressed/vmlinux
> 
> vmlinux is ELF image with ELF header. So actual point of kernel entry
> would be at an offset, somewhere after the ELF header.
> vmlinux.bin is what you would get after doing
> #objcopy -O binary vmlinux vmlinux.bin.
> vmlinux.bin has only obj code and nothing else.

To be more precise:

arch/x86/boot/compressed/vmlinux.bin is the result of
#objcopy -R .comment -S vmlinux
so it is still an ELF image

arch/x86/boot/vmlinux.bin is the result of:
#objcopy -O binary -R .note -R .comment -S arch/x86/boot/compressed/vmlinux

arch/x86/boot/compressed/vmlinux has the decompression stub + compressed
version of arch/x86/boot/compressed/vmlinux.bin

>>
>> In any event, it looks like either /arch/x86/boot/compressed/vmlinux.bin or
>> /vmlinux is what I need to copy into RAM @ 0x100000 (1MiB) which is where
>> my non-relocatable kernel is compiled to.
> 
> copy vmlinux.bin.

I don't think either is what I want - arch/x86/boot/vmlinux.bin still
contains a compressed kernel. And the contents of the compressed section is
an ELF image which requires more memcpys and memsets

What I really want, I think, is:

#objcopy -O binary -R .comment -S vmlinux

Which is not generated during the build process

>>  - How to setup the memory map (keeping in mind I have 2GB of contiguous
>>   memory with no BIOS/ACPI etc to worry about clobbering
>>  - Any other tricks I need to be aware of..
> Check if x86 kernel expects some parameters(like machineid, bootargs
> location etc) in some registers.
> Check if x86 has low level debug support(like DEBUG_LL).
> Also you might want to check how initial page tables are being setup in kernel.

I need to have a good look at what goes into arch/x86/boot/setup.bin - That
will really tell me what I need to do

Thanks,

Graeme