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

[Cornelia Huck]

On Thu, 27 Jun 2019 19:57:04 -0600
Alex Williamson <alex.williamson at redhat.com> wrote:

> 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.

Yes, I think it makes sense to fail the startup of a device where we
cannot set all attributes to the requested values.

> > 
> > 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"}
> >     ]

Modifying by index looks reasonable; I just sent a pull request to
print the index of an attribute out as well, so it is easier to specify
the right attribute to modify.

> > 
> > 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,  

What do you think of a way to specify JSON for the attributes directly
on the command line? Or would it be better to just edit the config
files directly?

> 
> 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

Fun ;)

> 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

I do not have ap devices to actually test this with; but defining a
device and adding attributes seems to work.

> need to check if this allows an NVIDIA vGPU mdev to be configured such
> that the framerate limiter can be automatically controlled.  Thanks,
> 
> Alex