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

From: George Dunlap <george.dunlap@citrix.com>
To: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Anthony Perard <anthony.perard@citrix.com>,
	Olaf Hering <olaf@aepfle.de>, Wei Liu <Wei.Liu2@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: Fri, 3 Jun 2016 11:13:07 +0100	[thread overview]
Message-ID: <57515833.1050003@citrix.com> (raw)
In-Reply-To: <22351.240.490849.992725@mariner.uk.xensource.com>

On 01/06/16 16:36, Ian Jackson wrote:
> George Dunlap writes ("Re: [Xen-devel] 4.7 qemu regression: HVM guests fail to boot from xvda"):
>> On Tue, May 31, 2016 at 12:41 PM, Olaf Hering <olaf@aepfle.de> wrote:
>>> 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.
> 
> Linux doctrine, at least, nowadays, is that hd* device names are not
> stable.  On my own colo machine I find that on some boots the actual
> hard disks are sda and sdb and the emergency usb stick is sdc, but on
> other boots the disks are sdb and sdc and the usb stick is sda.  So
> some would say that what you are doing is inherently unstable.
> 
> But I don't think I really agree with that view.  I think we should be
> able to do better.
> 
>> 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.
> 
> The vdev specifies the virtual block number.  See vbd-interface.txt.
> hda and xvda have different numbers.
> 
>>  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*.
> 
> Right.  This is the root cause.  vbd-interface.txt is designed to cope
> with modern PV frontends which never steal the hda minor numbers.
> 
> I think we should fix this with a syntax for explicitly specifying a
> pv-only device with the hd* pv block number.

Olaf,

Would this be a suitable solution for you, and do you think you
understand the suggestion well enough / have enough time to write up a
patch?

 -George

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