Re: [Xen-devel] [PATCH] libxl: Use xenbus to communicate with xenstore if the socket fails

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Thu, 2010-12-09 at 10:06 +0000, Vincent Hanquez wrote:
> On 09/12/10 09:17, Ian Campbell wrote:
> > On Wed, 2010-12-08 at 20:47 +0000, Mihir Nanavati wrote:
> >> Adds an open xenstore connection function which tries to use the
> >> xenbus interface (xs_domain_open) when the socket interface
> >> (xs_daemon_opn) fails.
> >
> > I like the concept of this patch.
> >
> > I can't think of any reason why the callers of libxenstore should need
> > to care about how which communication channel gets used so there's no
> > reason for the library to defer that choice to the caller. So perhaps we
> > should go one step further push this behaviour down into libxenstore
> > itself? i.e. add "xs_open(int ro)" and make
> > xs_{daemon,domain}_open{,_readonly} compatibility aliases to that
> > function.
> 
> Unfortunately, there is practical difference between the socket and the 
> page interface. Which is the reason why every xs connection in xapi are 
> sockets; IIRC polling for events on the page interface didn't really 
> work;

Good point.

The proposed function does try the socket connection first though so it
ought to work wherever it used too, at the expense of not getting a
useful failure message in cases where it wouldn't have.

The pvops kernels /proc/xen/xenbus driver has a poll method with
plausible looking runes in it so it's possible that the problem is
historical anyway. If that poll functionality turns out to be buggy then
we should just fix it anyway.

>  I think there was at least 1 other practical differences, but it 
> seems to have eluded me.

Ian.



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