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Subject: Re: [Xen-devel] [PATCH] xen: drop writing error messages to xenstore
To:     Juergen Gross <jgross@suse.com>,
        Joao Martins <joao.m.martins@oracle.com>
Cc:     linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org
References: <20181009160959.31076-1-jgross@suse.com>
 <5126873e-ade5-86b0-4ebf-58cb47c9cbe7@oracle.com>
 <90ae453c-f018-60d3-b7a9-e69cd39c0777@suse.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <b1e130bc-983f-f8df-0f5f-66971cc60454@oracle.com>
Date:   Wed, 10 Oct 2018 12:57:38 -0400
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Sender: linux-kernel-owner@vger.kernel.org
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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@suse.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.

-boris

>
> I believe you'll have to look into the guest anyway, so there is no need
> to have a message in the guest and one in Xenstore. The messages are
> text only and can be evaluated only if you know guest internals.
>
> blkfront for example prints an errno value into the message. I guess a
> windows guest wouldn't do that, or it could use other values for the
> same problem.
>
>
> Juergen