Re: [Xen-devel] Xen 4.2 Release Plan / TODO

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2012-04-12 at 11:24 +0100, Dario Faggioli wrote:
> On Tue, 2012-04-10 at 10:24 +0000, Ian Campbell wrote: 
> > Lots more DONE this week, still a few big ticket items or items with no
> > progress (that I've noticed) please let me know if the status of
> > anything has changed.
> >
> Here I am. :-)
> 
> > tools, blockers:
> >       * libxl stable API -- we would like 4.2 to define a stable API
> >         which downstream's can start to rely on not changing. Aspects of
> >         this are:
> >               * Safe fork vs. fd handling hooks. Involves API changes
> >                 (Ian J, patches posted)
> >               * locking/serialization, e.g., for domain creation. As of
> >                 now,  nothing is provided for this purpose, and
> >                 downstream toolstacks have to put their own mechanisms
> >                 in place. E.g., xl uses a fcntl() lock around the whole
> >                 process of domain creation. It should OTOH be libxl job
> >                 to offer a proper set of hooks --placed at the proper
> >                 spots during the domain creation process-- for the
> >                 downstreams to fill with the proper callbacks. (Dario
> >                 Faggioli)
> >                       * Thinking to defer this until 4.3?
> >
> Here's the point. This was raised for the following main reasons:
> 1. xl holds a lock during domain creation for way too long
> 2. checking for free memory in xl when it is Xen that will actually 
>     allocate it after a while is prone to races (the classical TOCTTOU 
>     condition)
> 
> We thought both these things to be addressable by messing around with
> locks, but a more careful analysis revealed we can't solve 2. in a sane
> enough way by just doing that (i.e., messing with lock, moving it
> around, etc.).
> 
> That being the case, yes, I think we can move this forward toward 4.3
> without much problems, and I propose doing so. Thoughts?

I think this is OK, it will effectively be a new API in 4.3 so it should
be reasonably easy to do in a compatible manner.

Ian.


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