Re: [Xen-devel] booting from HVM (?pv?)

[Alex Bligh <alex@xxxxxxxxxxx>]

--On 25 June 2013 12:57:59 +0200 AL13N <alien@xxxxxxxx> wrote:

> what i mean is:
>
> 1st try was using hda en hdb:cdrom with an iso and boot=dc
>
> this booted from the livecd, but the disk was not visible, even though
> xenblk_front was loaded (does hvm use a different module for emulation?
> there were some ide modules loaded as well).
This can happen if your live CD has one xen module but not the other
(xen_platform_pci and blkfrnot). It works like this. Early on kernel boot
the xen pci bridge module attempts to see if it has a connection to
the hypervisor. If it finds it does, it pokes a magic value to a port
(as George said) which unplugs the emulated hardware which has been
used by grub. sd may or may not have initialised (I believe depending
on whether it is a built in, whether the xen thing is built in, and
module init order - a.k.a. phase of the moon). Sometime later another
module (blkfront if I remember right) goes to see if there are any
pv driver disks it can talk to.

What this means in practice is that if you build your initrd without
blkfront support but with some of the other xen modules enabled, you
get the problem that you have no block devices on boot.

If you want some information on how this works, I suggest you read
this thread:
http://www.gossamer-threads.com/lists/xen/devel/192003

Ubuntu used to be broken in a slightly different way that may also
be relevant. See:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/804219


--
Alex Bligh

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