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

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Message-ID: <c86eadaf008f48aeb4bb7140a80b69e6@EX13D32EUC003.ant.amazon.com>
Date:   Mon, 9 Dec 2019 14:06:39 +0000
From:   "Durrant, Paul" <pdurrant@...zon.com>
To:     Jürgen Groß <jgross@...e.com>,
        Roger Pau Monné <roger.pau@...rix.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "Stefano Stabellini" <sstabellini@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: RE: [Xen-devel] [PATCH 2/4] xenbus: limit when state is forced to
 closed

> -----Original Message-----
> From: Jürgen Groß <jgross@...e.com>
> Sent: 09 December 2019 13:39
> To: Durrant, Paul <pdurrant@...zon.com>; Roger Pau Monné
> <roger.pau@...rix.com>
> Cc: linux-kernel@...r.kernel.org; xen-devel@...ts.xenproject.org; Stefano
> Stabellini <sstabellini@...nel.org>; Boris Ostrovsky
> <boris.ostrovsky@...cle.com>
> 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@...e.com>
> >> Sent: 09 December 2019 12:09
> >> To: Durrant, Paul <pdurrant@...zon.com>; Roger Pau Monné
> >> <roger.pau@...rix.com>
> >> Cc: linux-kernel@...r.kernel.org; xen-devel@...ts.xenproject.org;
> Stefano
> >> Stabellini <sstabellini@...nel.org>; Boris Ostrovsky
> >> <boris.ostrovsky@...cle.com>
> >> 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@...e.com>
> >>>> Sent: 09 December 2019 11:55
> >>>> To: Roger Pau Monné <roger.pau@...rix.com>; Durrant, Paul
> >>>> <pdurrant@...zon.com>
> >>>> Cc: linux-kernel@...r.kernel.org; xen-devel@...ts.xenproject.org;
> >> Stefano
> >>>> Stabellini <sstabellini@...nel.org>; Boris Ostrovsky
> >>>> <boris.ostrovsky@...cle.com>
> >>>> 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.

  Paul

> 
> 
> Juergen