qemu:///embed and isolation from global components

[Christophe de Dinechin]

Le 9 mars 2020 à 14:03, Michal Privoznik <mprivozn at redhat.com> a écrit :
> 
> On 3/6/20 6:49 PM, Daniel P. Berrangé wrote:
>>> On Fri, Mar 06, 2020 at 06:24:15PM +0100, Andrea Bolognani wrote:
>>> Recently I've been working on integrating qemu:///embed into an
>>> application. It's been reasonably smooth so far :)
>>> 
>>> There's one thing, however, that has caused a bit of confusion, and
>>> I would like to clarify whether my expectations are incorrect, there
>>> are genuine bugs in the implementation that need to be addressed, or
>>> maybe the experimental nature of embedded driver support results in
>>> not all scenarios having yet been considered and catered for.
>>> 
>>> Using virt-qemu-run as an example, when I run it as root I see that
>>> 
>>>   * the domain doesn't show up in the output of 'virsh list' for
>>>     qemu:///system;
>> This is good.
>>>   * it does, however, show up in the output of 'machinectl', with
>>>     class=vm and service=libvirt-qemu;
>> This is bad. It is one of the gaps we need to deal with.
>> A long while back I proposed a domain XML option for this:
>>   https://www.redhat.com/archives/libvir-list/2017-December/msg00729.html
>>     <resource register="none|direct|machined|systemd"/>
>> The "none" case meant inherit the cgroups placement of the parent
>> libvirtd (or now the app using the embedded driver), and would be
>> a reasonable default for the embedded case.
>> For the other cases, we certainly need to do something to ensure
>> uniqueness. This is possibly as simple as including a fixed
>> prefix like "qemu-$PID" where $PID is the app embedding the
>> driver. That said, we support closing the app, and re-starting
>> using the same embedded driver directory, which would change
>> PID.
> 
> Instead of PID we can derive the value from the root path, e.g. i-node of the root dir or some substring of the path hash, say first 6 letters of md5($root_path). This would guarantee the stability of the prefix across app restarts (if we assume all root dirs to be on one FS so we don't have clashing i-node numbers).
> 

Root path hash is more robust also in other cases, like file copy/rename often used for atomic updates. 

> Michal
>