Re: [PATCH v2] libxenstore: prefer using the character device

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2015-09-01 at 12:28 +0100, Ian Jackson wrote:
> David Vrabel writes ("Re: [Xen-devel] [PATCH v2] libxenstore: prefer 
> using the character device"):
> > On 27/08/15 19:03, Ian Jackson wrote:
> > > I confess I still see this as working around a kernel bug.  Only this
> > > time we are switching from a buggy to non-buggy kernel interface.
> > 
> > /proc/xen/xenbus is deprecated.  The tools should use the non
> > -deprecated
> > interface.
> 
> Why don't we just change it, then ?  What kernels don't provide
> /dev/xen/xenbus ?

It was added by 2fb3683e7b164ee2b324039f7c9d90fe5b1a259b, which looks to be
circa 3.2ish (git describe --contains gave me a stupid answer, so not sure
when it actually hit mainline, 3.2-rc was in Makefile at that commit).

It's perhaps nearly time but I don't think we can quite discard e.g. 3.0
yet and I'm not sure of the status of the forwarded ported classic Xen
kernels.

BTW around the same time /dev/xen/foo replacements were added for most
/proc/xen functionality, e.g. privcmd and xenbus_backend (replacing two
xsd_* I think). Really we ought to be moving over.

> > > > > +	if (access("/dev/xen/xenbus", F_OK) == 0)
> > > > > +		return "/dev/xen/xenbus";
> > > 
> > > Also, previously xs_domain_dev was a function which simply returned a
> > > static value.  I feel vaguely uneasy at putting this kind of
> > > autodetection logic here.
> > 
> > "Vaguely uneasy"?  Are we engineers or witchdoctors?
> 
> It doesn't fit my mental model of what this function is for.  I think
> it would be in better taste would be to arrange to attempt to call
> open() on both strings.  But perhaps that is too much to do at this
> stage of the release.
> 
> > xs_domain_dev() already does a system call to query the environment so
> > it did not just "return a static value":
> 
> getenv is not a system call.
> 
> 
> Anyway, if others don't have similar objections I am not nacking
> this.  It may be applied with Wei's ack.

I've done so, with my Ack as well. thanks.

Ian.