Re: [Xen-devel] [PATCH 2/4] xenbus: limit when state is forced to closed

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

> -----Original Message-----
> From: Jürgen Groß <jgross@xxxxxxxx>
> Sent: 09 December 2019 14:10
> To: Durrant, Paul <pdurrant@xxxxxxxxxx>; Roger Pau Monné
> <roger.pau@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx; Stefano
> Stabellini <sstabellini@xxxxxxxxxx>; Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH 2/4] xenbus: limit when state is forced to
> closed
> 
> On 09.12.19 15:06, Durrant, Paul wrote:
> >> -----Original Message-----
> >> From: Jürgen Groß <jgross@xxxxxxxx>
> >> Sent: 09 December 2019 13:39
> >> To: Durrant, Paul <pdurrant@xxxxxxxxxx>; Roger Pau Monné
> >> <roger.pau@xxxxxxxxxx>
> >> Cc: linux-kernel@xxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> Stefano
> >> Stabellini <sstabellini@xxxxxxxxxx>; Boris Ostrovsky
> >> <boris.ostrovsky@xxxxxxxxxx>
> >> Subject: Re: [Xen-devel] [PATCH 2/4] xenbus: limit when state is forced
> to
> >> closed
> >>
> >> On 09.12.19 13:19, Durrant, Paul wrote:
> >>>> -----Original Message-----
> >>>> From: Jürgen Groß <jgross@xxxxxxxx>
> >>>> Sent: 09 December 2019 12:09
> >>>> To: Durrant, Paul <pdurrant@xxxxxxxxxx>; Roger Pau Monné
> >>>> <roger.pau@xxxxxxxxxx>
> >>>> Cc: linux-kernel@xxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> >> Stefano
> >>>> Stabellini <sstabellini@xxxxxxxxxx>; Boris Ostrovsky
> >>>> <boris.ostrovsky@xxxxxxxxxx>
> >>>> Subject: Re: [Xen-devel] [PATCH 2/4] xenbus: limit when state is
> forced
> >> to
> >>>> closed
> >>>>
> >>>> On 09.12.19 13:03, Durrant, Paul wrote:
> >>>>>> -----Original Message-----
> >>>>>> From: Jürgen Groß <jgross@xxxxxxxx>
> >>>>>> Sent: 09 December 2019 11:55
> >>>>>> To: Roger Pau Monné <roger.pau@xxxxxxxxxx>; Durrant, Paul
> >>>>>> <pdurrant@xxxxxxxxxx>
> >>>>>> Cc: linux-kernel@xxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> >>>> Stefano
> >>>>>> Stabellini <sstabellini@xxxxxxxxxx>; Boris Ostrovsky
> >>>>>> <boris.ostrovsky@xxxxxxxxxx>
> >>>>>> Subject: Re: [Xen-devel] [PATCH 2/4] xenbus: limit when state is
> >> forced
> >>>> to
> >>>>>> closed
> >>>>>>
> >>>>>> On 09.12.19 12:39, Roger Pau Monné wrote:
> >>>>>>> On Thu, Dec 05, 2019 at 02:01:21PM +0000, Paul Durrant wrote:
> >>>>>>>> Only force state to closed in the case when the toolstack may
> need
> >> to
> >>>>>>>> clean up. This can be detected by checking whether the state in
> >>>>>> xenstore
> >>>>>>>> has been set to closing prior to device removal.
> >>>>>>>
> >>>>>>> I'm not sure I see the point of this, I would expect that a
> failure
> >> to
> >>>>>>> probe or the removal of the device would leave the xenbus state as
> >>>>>>> closed, which is consistent with the actual driver state.
> >>>>>>>
> >>>>>>> Can you explain what's the benefit of leaving a device without a
> >>>>>>> driver in such unknown state?
> >>>>>>
> >>>>>> And more concerning: did you check that no frontend/backend is
> >>>>>> relying on the closed state to be visible without closing having
> been
> >>>>>> set before?
> >>>>>
> >>>>> Blkfront doesn't seem to mind and I believe the Windows PV drivers
> >> cope,
> >>>> but I don't really understand the comment since this patch is
> actually
> >>>> removing a case where the backend transitions directly to closed.
> >>>>
> >>>> I'm not speaking of blkfront/blkback only, but of net, tpm, scsi,
> >> pvcall
> >>>> etc. frontends/backends. After all you are modifying a function
> common
> >>>> to all PV driver pairs.
> >>>>
> >>>> You are removing a state switc to "closed" in case the state was
> _not_
> >>>> "closing" before.
> >>>
> >>> Yes, which AFAIK is against the intention of the generic PV protocol
> >> such that it ever existed anyway.
> >>
> >> While this might be the case we should _not_ break any guests
> >> running now. So this kind of reasoning is dangerous.
> >>
> >>>
> >>>> So any PV driver reacting to "closed" of the other end
> >>>> in case the previous state might not have been "closing" before is at
> >>>> risk to misbehave with your patch.
> >>>
> >>> Well, they will see nothing now. If the state was not closing, it gets
> >> left alone, so the frontend shouldn't do anything. The only risk that I
> >> can see is that some frontend/backend pair needed a direct 4 -> 6
> >> transition to support 'unbind' before but AFAIK nothing has ever
> supported
> >> that, and blk and net crash'n'burn if you try that on upstream as it
> >> stands. A clean unplug would always set state to 5 first, since that's
> >> part of the unplug protocol.
> >>
> >> That was my question: are you sure all current and previous
> >> guest frontends and backends are handling unplug this way?
> >>
> >> Not "should handle", but "do handle".
> >
> > That depends on the toolstack. IIUC the only 'supported' toolstack is
> xl/libxl, which will set 'state' to 5 and 'online' to 0 to initiate an
> unplug.
> 
> I guess libvirt/libxl is doing the same?
> 

The unplug mechansism is all in libxl AFAICT, so it should be identical.

> At least at SUSE we still have some customers running xend based
> Xen installations with recent Linux or Windows guests.
> 

Is that something the upstream code can/should support though? I'd be surprised 
if xend is actually doing anything different to libxl since I've been coding 
the Windows PV drivers to trigger off the combined closing/online transition 
for as long as I can remember.

  Paul

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