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Re: [Xen-devel] [PATCH] xen: drop writing error messages to xenstore

  • To: Joao Martins <joao.m.martins@xxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
  • From: Juergen Gross <jgross@xxxxxxxx>
  • Date: Thu, 25 Oct 2018 14:36:59 +0200
  • Autocrypt: addr=jgross@xxxxxxxx; prefer-encrypt=mutual; keydata= xsBNBFOMcBYBCACgGjqjoGvbEouQZw/ToiBg9W98AlM2QHV+iNHsEs7kxWhKMjrioyspZKOB ycWxw3ie3j9uvg9EOB3aN4xiTv4qbnGiTr3oJhkB1gsb6ToJQZ8uxGq2kaV2KL9650I1SJve dYm8Of8Zd621lSmoKOwlNClALZNew72NjJLEzTalU1OdT7/i1TXkH09XSSI8mEQ/ouNcMvIJ NwQpd369y9bfIhWUiVXEK7MlRgUG6MvIj6Y3Am/BBLUVbDa4+gmzDC9ezlZkTZG2t14zWPvx XP3FAp2pkW0xqG7/377qptDmrk42GlSKN4z76ELnLxussxc7I2hx18NUcbP8+uty4bMxABEB AAHNHkp1ZXJnZW4gR3Jvc3MgPGpncm9zc0BzdXNlLmRlPsLAeQQTAQIAIwUCU4xw6wIbAwcL CQgHAwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJELDendYovxMvi4UH/Ri+OXlObzqMANruTd4N zmVBAZgx1VW6jLc8JZjQuJPSsd/a+bNr3BZeLV6lu4Pf1Yl2Log129EX1KWYiFFvPbIiq5M5 kOXTO8Eas4CaScCvAZ9jCMQCgK3pFqYgirwTgfwnPtxFxO/F3ZcS8jovza5khkSKL9JGq8Nk czDTruQ/oy0WUHdUr9uwEfiD9yPFOGqp4S6cISuzBMvaAiC5YGdUGXuPZKXLpnGSjkZswUzY d9BVSitRL5ldsQCg6GhDoEAeIhUC4SQnT9SOWkoDOSFRXZ+7+WIBGLiWMd+yKDdRG5RyP/8f 3tgGiB6cyuYfPDRGsELGjUaTUq3H2xZgIPfOwE0EU4xwFgEIAMsx+gDjgzAY4H1hPVXgoLK8 B93sTQFN9oC6tsb46VpxyLPfJ3T1A6Z6MVkLoCejKTJ3K9MUsBZhxIJ0hIyvzwI6aYJsnOew cCiCN7FeKJ/oA1RSUemPGUcIJwQuZlTOiY0OcQ5PFkV5YxMUX1F/aTYXROXgTmSaw0aC1Jpo w7Ss1mg4SIP/tR88/d1+HwkJDVW1RSxC1PWzGizwRv8eauImGdpNnseneO2BNWRXTJumAWDD pYxpGSsGHXuZXTPZqOOZpsHtInFyi5KRHSFyk2Xigzvh3b9WqhbgHHHE4PUVw0I5sIQt8hJq 5nH5dPqz4ITtCL9zjiJsExHuHKN3NZsAEQEAAcLAXwQYAQIACQUCU4xwFgIbDAAKCRCw3p3W KL8TL0P4B/9YWver5uD/y/m0KScK2f3Z3mXJhME23vGBbMNlfwbr+meDMrJZ950CuWWnQ+d+ Ahe0w1X7e3wuLVODzjcReQ/v7b4JD3wwHxe+88tgB9byc0NXzlPJWBaWV01yB2/uefVKryAf AHYEd0gCRhx7eESgNBe3+YqWAQawunMlycsqKa09dBDL1PFRosF708ic9346GLHRc6Vj5SRA UTHnQqLetIOXZm3a2eQ1gpQK9MmruO86Vo93p39bS1mqnLLspVrL4rhoyhsOyh0Hd28QCzpJ wKeHTd0MAWAirmewHXWPco8p1Wg+V+5xfZzuQY0f4tQxvOpXpt4gQ1817GQ5/Ed/wsDtBBgB CAAgFiEEhRJncuj2BJSl0Jf3sN6d1ii/Ey8FAlrd8NACGwIAgQkQsN6d1ii/Ey92IAQZFggA HRYhBFMtsHpB9jjzHji4HoBcYbtP2GO+BQJa3fDQAAoJEIBcYbtP2GO+TYsA/30H/0V6cr/W V+J/FCayg6uNtm3MJLo4rE+o4sdpjjsGAQCooqffpgA+luTT13YZNV62hAnCLKXH9n3+ZAgJ RtAyDWk1B/0SMDVs1wxufMkKC3Q/1D3BYIvBlrTVKdBYXPxngcRoqV2J77lscEvkLNUGsu/z W2pf7+P3mWWlrPMJdlbax00vevyBeqtqNKjHstHatgMZ2W0CFC4hJ3YEetuRBURYPiGzuJXU pAd7a7BdsqWC4o+GTm5tnGrCyD+4gfDSpkOT53S/GNO07YkPkm/8J4OBoFfgSaCnQ1izwgJQ jIpcG2fPCI2/hxf2oqXPYbKr1v4Z1wthmoyUgGN0LPTIm+B5vdY82wI5qe9uN6UOGyTH2B3p hRQUWqCwu2sqkI3LLbTdrnyDZaixT2T0f4tyF5Lfs+Ha8xVMhIyzNb1byDI5FKCb
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
  • Delivery-date: Thu, 25 Oct 2018 12:37:05 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

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@xxxxxxxx>
>>>>>> ---
>>>>>> 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, 
> 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.


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