Re: [PATCH 9/9] xenstore: when running in mini-os use printk for diagnostic messages

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 15/12/15 12:55, Juergen Gross wrote:
> On 15/12/15 13:52, Ian Campbell wrote:
>> On Tue, 2015-12-15 at 13:47 +0100, Juergen Gross wrote:
>>> On 15/12/15 13:31, Ian Campbell wrote:
>>>> On Fri, 2015-12-11 at 16:47 +0100, Juergen Gross wrote:
>>>>> Xenstore messages for diagnostic purposes are lost today in case
>>>>> xenstore is running under mini-os instead in a dom0 daemon.
>>>> Where does vfprintf end up under mini-os?
>>> TBH: I just couldn't find it. I tried to get some diagnostics when
>>> I started this work and couldn't find any place where the messages
>>> would occur.
>>>
>>>>> Use printk of mini-os in this situation.
>>>> and this now ends up on the console and (for a debug h/v) in the xen
>>>> dmesg,
>>>> is that right?
>>> The console of the xenstore domain seems to be a little bit special, as
>>> there is no component up to now writing the information needed to
>>> connect xenconsoled to the xenstore domain. This could be the reason I
>>> wasn't able to see any message printed via vfprintf. :-(
>> That sounds probable.
>>
>>> So in the end it will be part of xen dmesg.
>> Only for a debug=y Xen, not for a release build IIRC, unless you added some
>> other privilege along with your new flag which I missed.
> No, this is correct.
>
>>>  And this is where it should
>>> be, as such messages will be needed in case of xenstore domain not
>>> working properly. And I think it's console can be accessed by any means
>>> only in case of a working xenstore. :-)
>> An alternative would be init-xenstore-domain to daemonize and take on
>> respoibility for consuming the console ring and throwing it into a file?
> Interesting idea. I'll try this.

If the stubdomain could handle buffering of its ring for a while,
xenconsoled could just be used.  Alternatively, have xenconsoled able to
cope without xenstored while the xenstored stubdomain bootstraps itself.

IMO, it would be better to have just a single consoled, rather than
having a stub running alongside.  A stub would necessarily need some
negotiation with the main xenconsoled so they don't both map the console
ring and play the part of consumer.

~Andrew