Date:   Thu, 25 Oct 2018 14:36:59 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Joao Martins <joao.m.martins@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [Xen-devel] [PATCH] xen: drop writing error messages to xenstore

On 11/10/2018 13:03, Joao Martins wrote:
> On 10/11/2018 06:05 AM, Juergen Gross wrote:
>> On 10/10/2018 18:57, Boris Ostrovsky wrote:
>>> On 10/10/18 11:53 AM, Juergen Gross wrote:
>>>> On 10/10/2018 17:09, Joao Martins wrote:
>>>>> On 10/09/2018 05:09 PM, Juergen Gross wrote:
>>>>>> xenbus_va_dev_error() will try to write error messages to Xenstore
>>>>>> under the error/<dev-name>/error node (with <dev-name> something like
>>>>>> "device/vbd/51872"). This will fail normally and another message
>>>>>> about this failure is added to dmesg.
>>>>>>
>>>>>> I believe this is a remnant from very ancient times, as it was added
>>>>>> in the first pvops rush of commits in 2007.
>>>>>>
>>>>>> So remove the additional message when writing to Xenstore failed as
>>>>>> a minimum step.
>>>>>>
>>>>>> Signed-off-by: Juergen Gross <jgross@...e.com>
>>>>>> ---
>>>>>> I am considering removing the Xenstore write altogether, but I'm
>>>>>> not sure it isn't needed e.g. by xend based installations. So please
>>>>>> speak up in case you know why this write is there.
>>>>> So this:
>>>>>
>>>>> "This will fail normally and another message about this failure is added to dmesg."
>>>>>
>>>>> Brings me to the question: What about {stub,driver}domains? Ideally you
>>>>> shouldn't be looking at domU's dmesg as a control domain no? I can't remember
>>>>> any other error node, but if something fails e.g. netfront fails to allocate an
>>>>> unbound event channel - how do you know the cause from the control domain
>>>>> perspective?
>>>>>
>>>>> Irrespective of xend or not: isn't this 'error' node the only one that
>>>>> propagates error causes per device from domU?
>>>> What does it help you in dom0 if you have an error message in Xenstore
>>>> if a frontend driver couldn't do its job? Is there anything you can do
>>>> as a Xen admin?
>>>
>>> The admin may want to know, for example, that a hotplug in the guest failed.
>>
>> Shouldn't he ask the guest for that? There are dozens of other possible
>> problems letting hotplug fail which won't write anything to Xenstore.
>>
> But I think nothing stops people from using their own hotplug script that could
> use this error node (or even something else).
> 
>> This might be interesting for development/test purposes, but I really
>> question it to stay in mature code.
>>
> You're right in all of it: it doesn't convey the error in a agnostic manner, ATM
> doesn't report all errors involved in the device setup, and when a
> xenbus_dev_fatal happens you might end up looking at the guest anyways. But
> there might be users right now of this node e.g. cases where you have a bunch of
> known/trusted Linux guests working as backends (which also use this error node,
> it's not just frontends *I think*) which you would be able to recognize the
> error messages to inform the admin e.g. maybe QubesOS? It is merely an
> informative error message node, but it seems better than just a simple
> XenbusClosed state, with many reasons that it could lead to. Anyhow, just my 2c.

If there are any users this will be in rather old Xen versions only, as
writing the Xenstore node is _failing_ with newer Xen versions (that was
the original reason for writing that patch).

So back to my patch: any reason not to take it? After all it will only
remove the not very helpful kernel message that writing the Xenstore
node failed.


Juergen

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