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

["AL13N" <alien@xxxxxxxx>]

>
>
> --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.

thanks for the underlying technical details, in the past i've looked for
this stuff in documentation somewhere, (ie: how XENBUS handles the bus and
how guests need to react), but i didn't have much luck...

> 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.

xen_platform_pci -- i think we have this builtin in our kernel server...
ah wait, i forgot to doublecheck if server kernel was used...

i'm pretty sure we have blkfront and netfront in initrd...

> 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

i'll go read some more :-)


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