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

[Matthew Rosato]

On 6/28/19 5:06 AM, Cornelia Huck wrote:
> 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.
> 

I pulled and did a quick test with vfio-ap, it's working.  I was able to
define, modify with the appropriate attributes and start, resulting in a
correctly-configured device.