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: Mon, 6 Jun 2016 13:49:29 +0100	[thread overview]
Message-ID: <57557159.7010504@citrix.com> (raw)
In-Reply-To: <22353.28127.934683.210436@mariner.uk.xensource.com>

On 03/06/16 12:45, Ian Jackson wrote:
> George Dunlap writes ("Re: [Xen-devel] 4.7 qemu regression: HVM guests fail to boot from xvda"):
>> On 03/06/16 12:20, Olaf Hering wrote:
>>> I think the regression is: 'vdev=xvda' does not result in a disk
>>> connected to the emulated controller. Should we change the way hdtype=
>>> is handled internally? If hdtype= is not given it remains unset and with
>>> vdev=xvd* no disk-on-emulated-controller gets added. If hdtype= is set
>>> then vdev=xvd* will result in an disk-on-emulated-controller, which
>>> fixes the regression. If vdev=hd* and hdtype= was not set, hdtype will
>>> be silently set to ide.
>>
>> I'd be OK with this.  But is the "hdtype unset" also available at the
>> libxl level?
> 
> There are two problems with this `hdtype' approach.
> 
> Firstly, it is global.  That is, it applies to all disks of the
> particular guest.  But then maybe we don't care about that because
> this anomalous major-number-stealing behaviour is probably per-guest
> rather than per-disk.
> 
> Secondly, the proposal above involves changing both the semantics of
> existing `hdtype' parameter values, and the default hdtype value.  The
> resulting situation would be that even specifying vdev=hda wouldn't
> get you an emulated device, by default, unless you specified `hdtype'
> too.  I don't think that is right.
> 
> The possibilities I see are:
> 
> (1) New boolean per-guest parameter for this behaviour, meaning
>    `provide emulated devices for all xvd* as if they were hd*'.
> 
> (2) New `hdtype=ideforpv' which has the same effect as `hdtype=ide'
>    plus the semantics in (1) above.  (I'm open to better naming
>    suggestions.)
> 
> (3) New disk property parameter `hvm-emulate' in the Deprecated
>     section of xl-disk-configuration.txt.
> 
> Open questions:
> 
> Do we also need `... as if they were sd*' or `provide ide emulated
> devices where vdev=sd* is specified?'
> 
> If we have `hdtype=ideinclpv' do we also need `hdtype=ahciinclpv' ?
> 
> What should happen if these options are enabled for PV guests - should
> they be silently ignored, or should they be rejected ?

In general I like the fact that I can use the same config file and just
switch "builder" to either "generic" or "hvm" and have things Just Work.
 So I'd prefer to have things ignored.  (We might want to  thing about
whether we still want this to be done "silently" or not.)

 -George

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