Re: Users can provide their own kernels? (Was: Re: [Xen-users] Basic Question)

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> Very nice work!  Thanks.

You should thank Jeremy Katz (RedHat) for PyGrub (aq provided some additional 
patches,  IIRC - ReiserFS support, colour support, etc).

> Out of curiosity, does this mean that the domU kernel has to exist
> both inside the domU's root file system and in the dom0 filesystem?
> Or does Xen just start the bootloader from the domU filesystem and
> go from there?

Using PyGrub's approach, the bootloader is part of Xend and uses a filesystem 
library to poke into the guest filesystem and read the grub.conf / menu.lst 
and the kernel, initrd, etc.  The bootloader is running in dom0 but the 
kernel only needs to be in the guest filesystem.

Using the kexec approach, there'd be a bootloader kernel in dom0 that 
initially runs in the domain, mounts the FS and finds the appropriate files.  
Kexec is then used to jump into execution of a kernel from the guest 
filesystem.  Thus the bootloader runs in the domU *and* the guest kernel is 
in the domU filesystem.

The second approach is a bit more complicated to implement (from a developer 
PoV) but does have the advantages that all access to the guest filesystem 
occurs in an unprivileged domain and that it can immediately support all 
filesystems Linux will support.  *however* this will arguably be most 
important to people who are a) paranoid about security (highly untrusted 
guests) or b) use really weird filesystems ;-)

Cheers,
Mark

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