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

["Durrant, Paul" <pdurrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jürgen Groß <jgross@xxxxxxxx>
> Sent: 16 December 2019 08:10
> To: Durrant, Paul <pdurrant@xxxxxxxxxx>; David Miller
> <davem@xxxxxxxxxxxxx>
> 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 13.12.19 11:12, Durrant, Paul wrote:
> >> -----Original Message-----
> >> From: Jürgen Groß <jgross@xxxxxxxx>
> >> Sent: 13 December 2019 10:02
> >> To: Durrant, Paul <pdurrant@xxxxxxxxxx>; David Miller
> >> <davem@xxxxxxxxxxxxx>
> >> 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 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).
> >>
> >
> > But would someone sticking with such an old toolstack expect to run on
> an unmodified upstream dom0? There has to be some way in which we can
> retire old code.
> 
> As long as there are no hypervisor interface related issues
> prohibiting running dom0 unmodified I think the expectation to be
> able to use the kernel in that environment is fine.
> 

I think we need a better policy in future then otherwise we will only collect 
baggage.

> Another question coming up would be: how is this handled in a driver
> domain running netback? Which component is starting the hotplug script
> there? I don't think we can assume a standard Xen toolset in this case.
> So I'd rather leave this code as it is instead of breaking some rare
> but valid use cases.

I am not sure there is a standard. Do we 'support' driver domains with any sort 
of tools API or do they really just have to notice things via xenstore? I agree 
Linux running as a driver domain could indeed use udev.

> 
> >
> > Aside from the udev kicks though, I still think the hotplug-status/ring
> state interaction is just bogus anyway. As I said in a previous thread,
> the hotplug-status ought to be indicated as carrier status, if at all, so
> I still think all that code ought to go.
> 
> I agree regarding the future interface, but with the carrier state just
> being in the plans to be added now, it is clearly too early to remove
> the code with that reasoning.

I don't think so. Like I said, I think the hotplug status has nothing to do 
with the state of the shared ring. Even with the code as-is, nothing informs 
the frontend if the netif is subsequently closed or re-plumbed, so why must we 
continue to maintain this code? AFAICT it is just not fit for purpose.

  Paul
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