[libvirt] mdevctl: A shoestring mediated device management and persistence utility

[Alex Williamson]

On Thu, 27 Jun 2019 15:15:02 -0600
Alex Williamson <alex.williamson at redhat.com> wrote:

> On Thu, 27 Jun 2019 09:38:32 -0600
> Alex Williamson <alex.williamson at redhat.com> wrote:
> > > On 6/27/19 8:26 AM, Cornelia Huck wrote:    
> > > > 
> > > > {
> > > >   "foo": "1",
> > > >   "bar": "42",
> > > >   "baz": {
> > > >     "depends": ["foo", "bar"],
> > > >     "value": "plahh"
> > > >   }
> > > > }
> > > > 
> > > > Something like that?    
> > 
> > I'm not sure yet.  I think we need to look at what's feasible (and
> > easy) with jq.  Thanks,  
> 
> I think it's not too much trouble to remove and insert into arrays, so
> what if we were to define the config as:
> 
> {
>   "mdev_type":"vendor-type",
>   "start":"auto",
>   "attrs": [
>       {"attrX":["Xvalue1","Xvalue2"]},
>       {"dir/attrY": "Yvalue1"},
>       {"attrX": "Xvalue3"}
>     ]
> }
> 
> "attr" here would define sysfs attributes under the device.  The array
> would be processed in order, so in the above example we'd do the
> following:
> 
>  1. echo Xvalue1 > attrX
>  2. echo Xvalue2 > attrX
>  3. echo Yvalue1 > dir/attrY
>  4. echo Xvalue3 > attrX
> 
> When starting the device mdevctl would simply walk the array, if the
> attribute key exists write the value(s).  If a write fails or the
> attribute doesn't exist, remove the device and report error.
> 
> I think it's easiest with jq to manipulate arrays by removing and
> inserting by index.  Also if we end up with something like above, it's
> ambiguous if we reference the "attrX" key.  So perhaps we add the
> following options to the modify command:
> 
> --addattr=ATTRIBUTE --delattr --index=INDEX --value=VALUE1[,VALUE2]
> 
> We could handle it like a stack, so if --index is not supplied, add to
> the end or remove from the end.  If --index is provided, delete that
> index or add the attribute at that index.  So if you had the above and
> wanted to remove Xvalue1 but keep the ordering, you'd do:
> 
> --delattr --index=0
> --addattr --index=0 --value=Xvalue2
> 
> Which should results in:
> 
>   "attrs": [
>       {"attrX": "Xvalue2"},
>       {"dir/attrY": "Yvalue1"},
>       {"attrX": "Xvalue3"}
>     ]
> 
> If we want to modify a running device, I'm thinking we probably want a
> new command and options --attr=ATTRIBUTE --value=VALUE might suffice.
> 
> Do we need to support something like this for the 'start' command or
> should we leave that for simple devices and require a sequence of:
> 
> # mdevctl define ...
> # mdevctl modify --addattr...
> ...
> # mdevctl start
> # mdevctl undefine
> 
> This is effectively the long way to get a transient device.  Otherwise
> we'd need to figure out how to have --attr --value appear multiple
> times on the start command line.  Thanks,

This is now implemented, and yes you can specify '--addattr remove
--value 1' and mdevctl will immediately remove the device after it's
created (more power to the admin).  Listing defined devices also lists
any attributes defined for easy inspection.  It is also possible to
override the conversion of comma separated values into an array by
encoding and escaping the comma.  It's a little cumbersome, but
possible in case a driver isn't fully on board with the one attribute,
one value rule of sysfs.  Does this work for vfio-ap?  I also still
need to check if this allows an NVIDIA vGPU mdev to be configured such
that the framerate limiter can be automatically controlled.  Thanks,

Alex