Re: [Qemu-devel] [libvirt patch] qemu: adds support for virtfs 9p argument 'vii'

[Daniel P . Berrangé]

On Mon, May 06, 2019 at 07:58:28PM +0200, Christian Schoenebeck via Qemu-devel 
wrote:
> This is the counter part patch against latest libvirt git master head to
> support the 'vii' feature of patch 5, which introduces the XML config
> XML tag "important" on libvirt side.
> 
> To stick with the previous example mentioned with patch 5, likewise
> libvirt XML configuration might then look like this:
> 
>   <domain type='kvm'>
>     ...
>     <devices>
>       ...
>       <filesystem type='mount' accessmode='mapped'>
>         <source dir='/vm/fs'/>
>         <target dir='root'/>
>         <important path='/var/shares'/>
>         <important path='/tmp'/>
>       </filesystem>
>     </devices>
>   </domain>
> 
> Like with the vii qemu virtfs command line argument, the order of the
> "important" tag defines which one gets the highest inode namespace
> (smallest generated suffix) on guest side.

Do we think anyone is likely to use this feature in the real world ?

I'm not really a fan of the representation, because this is affecting
guest ABI via a side effect of the ordering which is the kind of thing
that has got us in trouble before.  If we need control over the IDs
used for each mount point, then I tend to think we need to represent
it explicitly such that the mgmt app sets the exact ID used.

     <pathid dir="/var/shares" id="0x1"/>
     <pathid dir="/tmp" id="0x3"/>

this ensures that the IDs are still stable when adding or removing
paths


Regards,
Daniel
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