linux-kernel - Re: [Xen-devel] Re: [PATCH 12/13] Unplug emulated disks and nics

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Date:	Thu, 08 Jul 2010 17:59:27 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"stefano@...bellini.net" <stefano@...bellini.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"sheng@...ux.intel.com" <sheng@...ux.intel.com>
Subject: Re: [Xen-devel] Re: [PATCH 12/13] Unplug emulated disks and nics

Ian Campbell wrote:
> On Thu, 2010-07-08 at 20:57 +0100, Don Dutile wrote:
>> I guess what I'm wondering is why not set xen_emul_unplug to ignore by
>> default (static int xen_emul_unplug=XEN_UNPLUG_IGNORE), which handles
>> the case I mentioned (just take existing guest config file as is, no edits,
>> pre-pv-hvm added to guest kernel),  and if person edits config file to 
>> change boot device to xvda, they would then edit the config to add
>> -x xen_emul_unplug=[all|ide-disks|...]  as well.
> 
> Can you guarantee that nobody is running an HVM guest today with a
> configuration file that specifies xvda (I believe it would work)? In
> other words can you be sure that defaulting to XEN_UNPLUG_IGNORE is
> _always_ going to be safe? Not just on RHEL hosts and with
> configurations generated by the RH tools or according to the RH docs but
> on any host with any (possibly hand-crafted) configuration?
> 
No, you have a valid point.  We have pv-on-hvm support for rhel3->rhel5
HVM guests, and they support xvda on boot devices (once initrd is rebuilt),
so it's possible to have that config (spec'd) as well, and someone
to copy & edit it for use on a more current disk image w/relatively current
kernel.
But I'm considering 2.6.32+ HVM guests that didn't have xvda spec'd in
the boot path ever, and are upgraded to a post-2.6.32 kernel that has
pv-on-hvm added to it.

> Any guest which uses xvda in its configuration file today will be using
> emulated devices but I think that with Stefano's patch and your proposed
> change in default on a Xen system without support for unplug will start
> using PV devices without unplugging the emulated ones first.
> 
Well, Stefano requires the admin to add unplug switch to kernel cmd line,
so I don't see the harm in defaulting to unplug... 

> I don't think there is any way for a guest running on a platform which
> does not support the unplug protocol to know automatically if it is safe
> to use the PV devices or not, therefore we have to err on the side of
> caution and ask users with such systems who know that their
> configuration is safe to explicitly request PV devices by using the
> command line option. Doing anything else is taking risks with people's
> data.
> 
> Ian.
> 
> 
Either way, the user/admin has to add cmdline to unplug to be safe.
I don't see how defaulting to UNPLUG_IGNORE changes that requirement.
... or am I missing a case? -- ah, if IGNORE isn't spec'd, pvhvm just won't
be configured in, and blkfront wont run, and cant have blkfront & ide
accessing the same device.... is that the case I'm missing ?

- Don


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