set custom loglevel for external libraries

[Jim Fehlig]

On 10/11/21 14:38, Olaf Hering wrote:
> Am Mon, 11 Oct 2021 13:25:20 -0600
> schrieb Jim Fehlig <jfehlig at suse.com>:
> 
>>>> I think it is wrong to use internal libvirt values for internal libxentoollog values.
>> What is gained over the sparse mapping? Are values not covered actually useful
>> in practice? Also, how are the values specified when using the xl tool? A quick
>> peek at the code seems to imply using more 'v' options. E.g. 'xl -vvvv ...'.
> 
> Since libvirt has no control about what libxentoollog values the authors
> use, it should provide a simple way to request a specific loglevel.
> xl decided to use the number of v's for it.

How does one correlate a specific log level to the correct number of 'v'? :-) 
IMO the sparse mapping we currently have is more intuitive.

>> If such a setting is actually needed, then the proper place would be
>> /etc/libvirt/libxl.conf. And IMO we should avoid creating a new name that could
>> potentially confuse users. If 'xentoollog_level' is the common jargon in xen, we
>> should stick with the same name.
> 
> If that is an acceptable approach I will try to prepare a patch.

I must admit I'm not fond of another log level knob. For many classes of bugs, 
users would then need to tweak libvirtd.conf and libxl.conf to collect 
appropriate debug info. That said, there is prior art. Going back to your 
snipped question

 >> I guess no other used library provides similar internal logging, this 
requirement is unique to libxenlight.so.

Sorry for not looking closer earlier, but the qemu driver has a similar setting 
in qemu.conf for the gluster libgfapi log level

https://gitlab.com/libvirt/libvirt/-/commit/a944bd925902d9ecce81e08900ad6a1adee06c6c

Jim