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

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Date:   Mon, 9 Dec 2019 14:38:44 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     "Durrant, Paul" <pdurrant@...zon.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

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".


Juergen