[Lguest] Host and Guests

[Rusty Russell]

On Monday 19 May 2008 21:45:50 Daniel Baluta wrote:
> I have another question regarding the next paragraph
>
> ===
>  Secondly, we only run specially modified Guests, not normal kernels.  When
>  you set CONFIG_LGUEST to 'y' or 'm', this automatically sets
>  CONFIG_LGUEST_GUEST=y, which compiles this file into the kernel so it
> knows how to be a Guest.  This means that you can use the same kernel you
> boot normally (ie. as a Host) as a Guest.
> ===
>
> but CONFIG_LGUEST = y is also set within Host
> as i have seen in http://lguest.ozlabs.org/lguest.txt , configuring kernel.
>
> so where is the difference between a host and a guest, regarding the
> part when each one finds its identity ( ex: i'm host or guest?)

This has changed in more recent kernels (you can adjust the guest and host 
config options separately now).

The method by which we know we're a guest changed.  In 2.6.23, the Launcher 
entered the Guest immediately after the string "GenuineLguest" which was 
found in lguest_asm.S: this jumps to lguest_init.

In 2.6.24, the bootloader protocol was extended, and now the Guest is launched 
like a normal kernel.  From arch/x86/lguest/i386_head.S:

/*G:020 Our story starts with the kernel booting into startup_32 in
 * arch/x86/kernel/head_32.S.  It expects a boot header, which is created by
 * the bootloader (the Launcher in our case).
 *
 * The startup_32 function does very little: it clears the uninitialized
 * global C variables which we expect to be zero (ie. BSS) and then copies the
 * boot header and kernel command line somewhere safe.  Finally it checks the
 * 'hardware_subarch' field.  This was introduced in 2.6.24 for lguest and
 * Xen: if it's set to '1' (lguest's assigned number), then it calls us here.

All this information can be found in "make Preparation" in the drivers/lguest 
directory...

Hope that helps,
Rusty.