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

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Message-ID: <alpine.DEB.2.00.1007091150000.21432@kaball-desktop>
Date:	Fri, 9 Jul 2010 11:54:24 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Ian Campbell <Ian.Campbell@...citrix.com>
CC:	"ddutile@...hat.com" <ddutile@...hat.com>,
	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

On Fri, 9 Jul 2010, Ian Campbell wrote:
> > 
> > > 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,
> 
> In the case where the host platform does not support the unplug protocol
> this is correct and requiring explicit admin action to allow the PV
> frontends to activate is the only safe option WRT the users data.
> 
> However if the host platform does support the unplug protocol then this
> is incorrect. In that case the default (in the absence of the command
> line option) is to automatically unplug any device for which a PV driver
> is available and so no command line option will be required in the
> common case. (see xen_unplug_emulated_devices() under the comment "Set
> the default value of xen_emul_unplug depending on...")
> 

that's right: on host platforms supporting unplug no command line options
are required; on the other hand if the host platform does not support
unplug then xen_emul_unplug=ignore is required to use PV drivers anyway.


> > so I don't see the harm in defaulting to unplug... 
> 
> As I described in my previous mail this is unsafe on host platforms
> which do not support unplug. As I describe above it is unnecessary on
> host platforms which do support unplug
> 

yep, keep in mind the copy and paste example.



If you are happy about the patch, I'll include it in my next version.

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