Re: 4.7 qemu regression: HVM guests fail to boot from xvda

From: George Dunlap <george.dunlap@citrix.com>
To: Olaf Hering <olaf@aepfle.de>, Wei Liu <Wei.Liu2@citrix.com>
Cc: Anthony Perard <anthony.perard@citrix.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>
Subject: Re: 4.7 qemu regression: HVM guests fail to boot from xvda
Date: Tue, 31 May 2016 13:00:45 +0100	[thread overview]
Message-ID: <CAFLBxZYMAMiH8WweWeNxee7r5h8EZRK-LRM37e=34OGvHEHb6A@mail.gmail.com> (raw)
In-Reply-To: <20160531114141.GA17626@aepfle.de>

On Tue, May 31, 2016 at 12:41 PM, Olaf Hering <olaf@aepfle.de> wrote:
> On Tue, May 31, George Dunlap wrote:
>
>> Sorry, can you expand on this a bit?  Are you saying that on SuSE, if
>> you specify "vdev=xvda" in your config file, that you'll get PV
>> devices named "/dev/xvda", but that if you specify "vdev=hda", that
>> you'll get PV devices but named "/dev/hda"?
>
> Yes, thats exactly what the xenlinux block frontend does.
> pvops forces xvda, independent of the name 'vdev' in domU.cfg.
> Up to xen-4.2 'vdev=hd*' was required to tell qemu to create an emulated
> disk to boot from. Starting with xen-4.3 qemu also recognized
> 'vdev=xvd*' for the emulated disk. And starting with xen-4.7 qemu
> requires 'xvda=hd*' again.
>
> I think if some domU.cfg for xen-4.3+ has 'vdev=xvd*' and the domU uses
> for some reason kernel names in config files and the domU uses a
> xenlinux kernel, then changing domU.cfg to 'hd*' will allow the guest to
> boot again. But its userland will miss the /dev/xvd* device nodes.
> That probably remained unnoticed during testing the referenced commit if
> a pvops based kernel was used.

Or if -- as is the case for most of my own test systems -- filesystem
UUIDs are used rather than device names.  (This means things work the
same on PV with PV disks, HVM with PV disks, and HVM with emulated
disks -- for instance, if you're using nested virtualization and your
L1 dom0 can't access L0 xenbus.)

Do you have a concrete proposal?

Anthony, does the OVMF-with-pv-only-drivers actually still work at the moment?

Really 'vdev' string in the the guest config file is only meant to
tell libxl how it should behave -- it should ideally not have any
effect on what devices you see in the backend.  And furthermore, it
seems to me that when Linux upstream rejected the idea of the pv
drivers stealing the "hd*" namespace, that SuSE's xenlinux should have
followed suit and had the pv drivers only create devices named xvd*.

But I recognize if you're selling an enterprise kernel, those sorts of
"you should have done this X years ago" doesn't really help you keep
your promises to your users now. :-)

 -George

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