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

[Alex Williamson]

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,

Alex