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

[Cornelia Huck]

On Wed, 26 Jun 2019 19:53:50 -0600
Alex Williamson <alex.williamson at redhat.com> wrote:

> On Wed, 26 Jun 2019 08:37:20 -0600
> Alex Williamson <alex.williamson at redhat.com> wrote:
> 
> > On Wed, 26 Jun 2019 11:58:06 +0200
> > Cornelia Huck <cohuck at redhat.com> wrote:
> >   
> > > On Tue, 25 Jun 2019 16:52:51 -0600
> > > Alex Williamson <alex.williamson at redhat.com> wrote:
> > >     
> > > > Hi,
> > > > 
> > > > Based on the discussions we've had, I've rewritten the bulk of
> > > > mdevctl.  I think it largely does everything we want now, modulo
> > > > devices that will need some sort of 1:N values per key for
> > > > configuration in the config file versus the 1:1 key:value setup we
> > > > currently have (so don't consider the format final just yet).      
> > > 
> > > We might want to factor out that config format handling while we're
> > > trying to finalize it.
> > > 
> > > cc:ing Matt for his awareness. I'm currently not quite sure how to
> > > handle those vfio-ap "write several values to an attribute one at a
> > > time" requirements. Maybe 1:N key:value is the way to go; maybe we
> > > need/want JSON or something like that.    
> > 
> > Maybe we should just do JSON for future flexibility.  I assume there
> > are lots of helpers that should make it easy even from a bash script.
> > I'll look at that next.  
> 
> Done.  Throw away any old mdev config files, we use JSON now. 

The code changes look quite straightforward, thanks.

> The per
> mdev config now looks like this:
> 
> {
>   "mdev_type": "i915-GVTg_V4_8",
>   "start": "auto"
> }
> 
> My expectation, and what I've already pre-enabled support in set_key
> and get_key functions, is that we'd use arrays for values, so we might
> have:
> 
>   "new_key": ["value1", "value2"]
> 
> set_key will automatically convert a comma separated list of values
> into such an array, so I'm thinking this would be specified by the user
> as:
> 
> # mdevctl modify -u UUID --key=new_key --value=value1,value2

Looks sensible.

For vfio-ap, we'd probably end up with something like the following:

{
  "mdev_type": "vfio_ap-passthrough",
  "start": "auto",
  "assign_adapter": ["5", "6"],
  "assign_domain": ["4", "0xab"]
}

(following the Guest1 example in the kernel documentation)

<As an aside, what should happen if e.g "assign_adapter" is set to
["6", "7"]? Remove 5, add 7? Remove all values, then set the new ones?
Similar for deleting the "assign_adapter" key. We have an
"unassign_adapter" attribute, but this is not something we can infer
automatically; we need to know that we're dealing with an vfio-ap
matrix device...>

> 
> We should think about whether ordering is important and maybe
> incorporate that into key naming conventions or come up with some
> syntax for specifying startup blocks.  Thanks,
> 
> Alex

Hm...

{
  "foo": "1",
  "bar": "42",
  "baz": {
    "depends": ["foo", "bar"],
    "value": "plahh"
  }
}

Something like that?