Re: booting from HVM (?pv?)

[George Dunlap <george.dunlap@eu.citrix.com>]

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@rmail.be> 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