Re: [Xen-devel] [RFC][PATCH 0/5] Add V4V to Xen

[Jean Guyader <jean.guyader@xxxxxxxxxx>]

On 07/06 04:36, Tim Deegan wrote:
> At 12:40 +0100 on 07 Jun (1339072831), Tim Deegan wrote:
> > Hi, 
> > 
> > Thanks for this.
> > 
> > Overall, it looks like Xen is doing a few things here:
> >  - nameservice for registering services and finding endpoints;
> >  - ring manipulation arithmetic;
> >  - copying data; and
> >  - notifying endpoints. 
> > 
> > The shared-ring logic was able to do all of these, with a few drawbacks:
> >  - The xenstore handshake stuff is really grotty;
> >  - grant maps can cause zombie domains; and
> >  - it doesn't do many-to-one multicast.
> 
> We've just had a discussion of this in person, and from my notes the two
> things that stand out are:
> 
>  - yes, you want to do many-to-one multiplexing, in particular to 
>    avoid the server needing a ring for every client; and
>  - one reason not to use Xenstore is that there is only one Xenstore
>    page per VM and it may not be possible to safely share it with other
>    users (in particular between a BIOS/EFI user and the main OS).
> 
> The Xenstore point is understandable, and probably something that ought
> to be fixed anyway -- we have seen people run into similar problems with
> BIOS drivers for blkfront and netfront.
> 
> Using one ring for all clients raises the question of access control and
> admission control -- in particular how do you avoid DoS from
> badly-behaved clients?
> 
> And, given your concerns about sharing an OS with an uncooperative
> Xenstore client, how do you handle sharing the OS with a badly behaved
> v4v client?
> 
> If we _do_ need one ring with multiple writers, and therefore Xen needs
> to arbitrate writes, there's still room for the policy-based parts
> (controlling connection setup, for example) to live outside the
> hypervisor, openvswitch-style.
> 

Thanks for summary Tim.

Today the acl check in V4V (not part of the current patch serie) is done
for every copy by Xen. Moving the policy control outside of Xen would mean that
you still need to have a copy of the acls in Xen and the worst
thing that can happen is for the copy to get out of sync.

What do you think would be the next step going forward?

Jean

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