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

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Message-ID: <4C372743.6070003@redhat.com>
Date:	Fri, 09 Jul 2010 09:42:27 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Ian Campbell <Ian.Campbell@...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

Stefano Stabellini wrote:
> 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.
> 
> 
I see by looking at xen_unplug_emulated_devices() closer, that the default
unplugs emulated ide & emulated nic:
	if (xen_platform_pci_enabled && !xen_emul_unplug)

I had to work around the fact that xen_platform_pci_enabled would
not be set on rhel5-dom0 (no emul-unplug), so I hadn't looked at this default
so closely.


>>> 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.
> 

Yes, the patch seems like a reasonable solution for running on older qemu-dm's.

Thanks for the clarification(s).

- Don
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