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

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 06/25/2013 11:57 AM, AL13N wrote:
> > On 06/25/2013 11:47 AM, AL13N wrote:
> > > > On Mon, Jun 24, 2013 at 9:21 PM, AL13N <alien@xxxxxxxx> wrote:
> > > > > Hi,
> > > > >
> > > > > I'm the Mageia XEN packager and during QA, we stumbled into a problem.
> > > > >
> > > > > in fact, we wanted to test Mageia 3 installation on a HVM.
> > > > >
> > > > > so, we had a sparse image and a iso file:
> > > > >
> > > > > [ 'file:/opt/testhvm.img,sda,w',
> > > > > 'file:/opt/mageialive.iso,hdb:cdrom,r'
> > > > > ]
> > > > >
> > > > > the live booted, and was able to install to disk, but it never seemed
> > > > > to
> > > > > boot
> > > > > after the install... (for some reason)
> > > > >
> > > > > in the end, this "worked" when we changed to:
> > > > >
> > > > > [ 'file:/opt/testhvm.img,xvda,w', 'file:/opt/testhvm.img,sda,w' ]
> > > > >
> > > > > apparently 'xvda' to get grub to boot the kernel; and 'sda' so that it
> > > > > could
> > > > > start...
> > > > My HVM Linux config files have 'hda' instead of 'sda' -- can you try
> > > > that instead?
> > > >
> > > > Normally what happens is that qemu begins by exposing the hda device
> > > > to the guest, to boot via grub; but when the Xen PV drivers in the
> > > > Linux kernel come up, they write to a magic port which causes the
> > > > physical hda device to disappear.  I *think* then that the Xen PV
> > > > drivers actually take over that major/minor, so that further reads and
> > > > writes to the hda go through the PV protocol instead.
> > > >
> > > > All of this might get mixed up if you're using sda instead.
> > > >
> > > >    -George
> > >
> > > well, hda is what we tried first, but we couldn't see the disk from the
> > > live system with hda. only sda seemed to work for that...
> > When you say, "we couldn't see the disk from the live system", you're
> > talking about booting from the live CD?
> >
> > If post-install you try 'hda' and it boots (after changing the grub and
> > /etc/fstab if necessary), then I would suspect that there's a problem
> > with your live CD kernel.  Do you use a different kernel image, and/or
> > are the modules for the Xen PV devices not included?
> what i mean is:
>
> 1st try was using hda en hdb:cdrom with an iso and boot=dc
Hmm, I think we normally use hdc for the cdrom instead of hdb.  hda and 
hdb would be on the same controller, I think; so it's probably not 
possible to unplug hda without also unplugging hdb.  In that case, 
having the PV drivers loaded could actually make things worse: if they 
successfully unplug hda, then they lose the cdrom; if they don't, then 
the PV drivers may be in a weird state where they've grabbed the hda 
device but aren't able to provide access to the disk anymore.

> 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 is one thing we wanted to get right when having Mageia to be
> xen-ready (ie: installable on HVM from iso)
Absolutely -- I'm just exploring configuration / guest kernel setups 
that can be tweaked, particularly since at the moment we're pretty much 
locked down for 4.3, so any changes won't be in a public release until 
4.4 in (probably) 6 months' time.
> We'll try using hda postinstall...
>
>
> PS: one weird thing that i saw in the log files, was a message about
> stripping tap from device, which was odd, since file:/... was specified
> and not tap:aio:... (and Mageia 3 doesn't have a specific -xen kernel, nor
> a blktap-dkms...)
If you could post the exact message, we could find out where the warning 
was coming from and see if it's something to worry about.
 -George

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