[Xen-devel] Re: [Xen-changelog] Simpler domid allocation.

[Anthony Liguori <aliguori@xxxxxxxxxx>]

Keir Fraser wrote:

> I think you misread the allocation scheme -- it still allocates 
> increasing domid's in turn but just doesn;t arbitrarily decide to wrap 
> at e.g., domid==100. So your fears over immediate domid reuse are 
> unfounded.
Indeed. I read the removed comment as the new functionality. Should not 
read patches before having any caffeine :-)
> I understand your fears w.r.t. decentralised tools. One thing I think 
> we will end up doing is adding the domain uuid (big unique domain 
> identifier) to Xen. Xen will still work in terms of the short 16-bit 
> domids, but control tools will be able to read out this extra unique 
> key value which they can use to protect themselves against domid reuse 
> in the absence of some other trusted authority. Invaluable for 
> debugging screwed-up machine states too. :-)
Excellent. The UUID would be part of the struct domain in Xen? Would Xen 
create it and the tools just have an interface to get it? I'd be happy 
to submit a patch to add this as it would be very useful for the tools 
(since it effectively eliminates the possibility of race conditions).
Thanks,

Anthony Liguori

> -- Keir
>
> On 15 Jul 2005, at 15:40, Anthony Liguori wrote:
>
> > This change worries me a bit because I believe it increases the 
> > likelihood of subtle race conditions in the tools. It's now likely 
> > that if one destroys a domain and immediately creates a new domain, 
> > when the tools finally see the VIRQ, they'll be very confused since 
> > the domid appears to be valid.
> > The situation is worse for a destroyed domain. A destroy does not 
> > generate a VIRQ in which case if the destroy and create happen within 
> > whatever the tools polling interval is, it will appear to the tool 
> > that the destroy just didn't work.
> > The solution would be to have a completely serialized tool chain 
> > although that prevents having multiple simulatenous tools running at 
> > the same time or a distribute tool chain where most things don't 
> > require a central daemon.
> > The old domid allocation was odd but it kept life easier by making 
> > race conditions difficult to create.

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