Re: [Xen-devel] [Hackathon Minutes] Xen 4.4 Planning

[Alex Bligh <alex@xxxxxxxxxxx>]

--On 14 June 2013 13:25:38 -0400 Konrad Rzeszutek Wilk 
<konrad.wilk@xxxxxxxxxx> wrote:
> What we had issues was that when compiling against a new hypervisor
> API it all compiled, but launching guests was broken. What we found was
> that some structures had changed and their corresponding
> XEN_VERSION_SOMETHING changed as well. But it was not clear _which_ of
> the structures changed so it took a while to figure out that the cpumap
> had been changed (I think that is what it was). There was also some HVM
> parameter field that had to be added otherwise the guest would not boot
> (can't remember the details).
That's precisely it.

Actually the one that REALLY bit us was far more subtle. We have a
multithreaded, forking agent. In Xen 4.2 (but not 4.1) you have to
call some post-fork function (whose name escapes me) in order to
tell xen about a fork. Signal handlers have to be in a particular
state (in particular SIGCHLD being set to ignore causes subtle
problems). And to get various xl calls working (mainly server
creation), it turns out you need to do a fork (assuming your
client is multithreaded and other threads call other xen things
even with different contexts); again this wasn't necessary in 4.1.

We have this working reliably now, but it was a voyage of
discovery particularly as there are no multithreaded client examples,
and whilst this is (sort of) explained in the header files, it's a
bit opaque. Seeing xl launch stuff, and our agent fail when it was
passing byte for byte the same values was frustrating. If I felt I
really understood how this worked (rather than discovered from trial
and error and a lot of strace), I'd have sent a documentation patch.

So this wasn't a change in the API in the sense of structs changing
or parameters changing (we caught that early at compile time), but
in how the thing gets used in practice.

I fully understand why the API was changed 4.2 -> 4.3, and really
appreciate the fact it's now stable (or at least advertised as
such). Initial indications (i.e. it built first time without
warnings) are good.

--
Alex Bligh

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