Re: [xen-devel][PATCH] PV driver compatibility

["Ky Srinivasan" <ksrinivasan@xxxxxxxxxx>]

>>> On 1/21/2010 at  3:38 PM, in message
<  1264106300.21707.57.camel@xxxxxxxxxxxxxxxxxxxxx  >, Ian Campbell
<  Ian.Campbell@xxxxxxxxxx  > wrote: 
> On Wed, 2010-01-20 at 16:07 +0000, Keir Fraser wrote: 
>> On 20/01/2010 16:02, "Ky Srinivasan" <  ksrinivasan@xxxxxxxxxx  > wrote:
>> 
>> > The attached patch fixes what I believe is a typo and permits guests 
> running
>> > the latest PV drivers to correctly interact with older back-ends.
>> > 
>> > Signed-off-by: K. Y. Srinivasan <  ksrinivasan@xxxxxxxxxx  >
>> 
>> Ian,
>> 
>> You introduced the magic port value check, in xen-unstable:19964.
> 
> I'm guilty of pretty poor changelogging there, aren't I, I've no idea
> how the unmodified drivers part of the change relates to the comment :-(
> 
>> Can you ack/nack this please?
> 
> What vintage of older back-ends are we talking about?
We shipped sles11 on xen-3.3.1 and this is where we encountered the problem - 
hosting a sles11 sp1 (based on xen-4.0.0)  guest on a sles11  host. 
> 
> What is their behaviour when reading from that port? Can we test for a
> specific value instead of anything != MAGIC or is there some other way
> to identify them? 

Looking at your documentation for this new protocol, I recall seeing that if 
the magic value was not read, it was ok to silently return to be compatible.
> 
> Without some sort of unplugging mechanism we run the risk of having both
> PV and Emulated disk controllers active, accessing the same virtual disk
> and with drivers loaded in the guest, which is potentially very
> dangerous for the user's data. Did those older backends implement some
> alternative unplugging mechanism we should be trying?
We have had a mechanism for disabling the emulation when the PV drivers are 
loaded for some time now.
> 
> The whole point of this magic check is to ensure we are running on a
> backend which is new enough to do the unplugging in a safe way, so I
> think failing to switch to PV and sticking with emulated on such
> platforms the safe approach.

This breaks the compatibility with systems that have already been shipped in 
the sense we cannot run the guests with PV drivers. The proposed patch fixes 
the problem.

Regards,

K. Y
> 
> I'd suggest that this issue should be fixed by backporting the backend
> support for the safe unplug protocol -- I don't think the patch for such
> minimal support is that big or risky. Failing that perhaps it could at
> least be something you need to explicitly ask for if you have somehow
> verified that there is no danger of the emulated and PV backends
> trampling each other, e.g. via a frontend module parameter.
> 
> Ian.




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