Re: [Xen-devel] [PATCH net-next] xen-netback: get rid of old udev related code

[Jürgen Groß <jgross@xxxxxxxx>]

On 13.12.19 10:24, Durrant, Paul wrote:
> > -----Original Message-----
> > From: Jürgen Groß <jgross@xxxxxxxx>
> > Sent: 13 December 2019 05:41
> > To: David Miller <davem@xxxxxxxxxxxxx>; Durrant, Paul
> > <pdurrant@xxxxxxxxxx>
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; wei.liu@xxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> > Subject: Re: [Xen-devel] [PATCH net-next] xen-netback: get rid of old udev
> > related code
> >
> > On 12.12.19 20:05, David Miller wrote:
> > > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> > > Date: Thu, 12 Dec 2019 13:54:06 +0000
> > >
> > > > In the past it used to be the case that the Xen toolstack relied upon
> > > > udev to execute backend hotplug scripts. However this has not been the
> > > > case for many releases now and removal of the associated code in
> > > > xen-netback shortens the source by more than 100 lines, and removes
> > much
> > > > complexity in the interaction with the xenstore backend state.
> > > >
> > > > NOTE: xen-netback is the only xenbus driver to have a functional
> > uevent()
> > > >         method. The only other driver to have a method at all is
> > > >         pvcalls-back, and currently pvcalls_back_uevent() simply returns
> > 0.
> > > >         Hence this patch also facilitates further cleanup.
> > > >
> > > > Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
> > > If userspace ever used this stuff, I seriously doubt you can remove this
> > > even if it hasn't been used in 5+ years.
> > Hmm, depends.
> >
> > This has been used by Xen tools in dom0 only. If the last usage has been
> > in a Xen version which is no longer able to run with current Linux in
> > dom0 it could be removed. But I guess this would have to be a rather old
> > version of Xen (like 3.x?).
> >
> > Paul, can you give a hint since which Xen version the toolstack no
> > longer relies on udev to start the hotplug scripts?
> The udev rules were in a file called tools/hotplug/Linux/xen-backend.rules (in 
> xen.git), and a commit from Roger removed the NIC rules in 2012:
>
> commit 57ad6afe2a08a03c40bcd336bfb27e008e1d3e53
Xen 4.2

> The last commit I could find to that file modified its name to 
> xen-backend.rules.in, and this was finally removed by George in 2015:
>
> commit 2ba368d13893402b2f1fb3c283ddcc714659dd9b
Xen 4.6

> So, I think this means anyone using a version of the Xen tools within recent 
> memory will be having their hotplug scripts called directly by libxl (and 
> having udev rules present would actually be counter-productive, as George's 
> commit states and as I discovered the hard way when the change was originally 
> made).
The problem are systems with either old Xen versions (before Xen 4.2) or
with other toolstacks (e.g. Xen 4.4 with xend) which want to use a new
dom0 kernel.

And I'm not sure there aren't such systems (especially in case someone
wants to stick with xend).


Juergen

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