Kernel using LZMA compression

Bryn M. Reeves bmr at redhat.com
Wed Nov 4 19:33:40 UTC 2009


On 11/04/2009 06:37 PM, Ikem Krueger wrote:
>> I am reading between the lines here (I have never looked at this
>> stuff in Xen) but I would assume it's for the reason given above.
>> The kernel's own decompression routines must run very early on in
>> the boot process - well before the first line of C code runs and
>> while the CPU (on x86) is still running in legacy real addressing
>> mode (right after the handover from the bootloader and relocation
>> of the kernel image).
>
> Ok. Sounds plausible. How is it to seperate the routines? Can they
> brought from "legacy mode" to "real mode"?
>

Quite tricky I'd guess - it's chicken-and-egg. The code to switch the
CPU from real mode to protected mode is in the kernel's startup routines
*inside* the compressed image.

I don't think anyone is going to want to reorganise things to move that 
code to the primitive early-boot period - the idea is to do as little as 
possible in that part of the kernel and leave everything else to later 
in the boot process when life gets easier.

Decompressing the kernel is always going to be done in that part of the 
startup sequence because that's when it has to happen.

Regards,
Bryn.




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