Re: mdevctl: A shoestring mediated device management and persistence utility

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 6/27/19 11:38 AM, Alex Williamson wrote:
> On Thu, 27 Jun 2019 11:00:31 -0400
> Matthew Rosato <mjrosato@linux.ibm.com> wrote:
> 
>> On 6/27/19 8:26 AM, Cornelia Huck wrote:
>>> On Wed, 26 Jun 2019 19:53:50 -0600
>>> Alex Williamson <alex.williamson@redhat.com> wrote:
>>>   
>>>> On Wed, 26 Jun 2019 08:37:20 -0600
>>>> Alex Williamson <alex.williamson@redhat.com> wrote:
>>>>  
>>>>> On Wed, 26 Jun 2019 11:58:06 +0200
>>>>> Cornelia Huck <cohuck@redhat.com> wrote:
>>>>>     
>>>>>> On Tue, 25 Jun 2019 16:52:51 -0600
>>>>>> Alex Williamson <alex.williamson@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?  
>>
>> IMO remove 5, add 7 would make the most sense.  I'm not sure that doing
>> an unassign of all adapters (effectively removing all APQNs) followed by
>> an assign of the new ones would work nicely with Tony's vfio-ap dynamic
>> configuration patches.
> 
> Are we conflating operating on the config file versus operating on the
> device?  I was thinking that setting a new key value replaces the
> existing key, because anything else adds unnecessary complication to
> the code and command line.  So in the above example, if the user
> specified:
> 
>   mdevctl modify -u UUID --key=assign_adapter --value=6,7
> 
> The new value is simply ["6", "7"].  This would take effect the next
> time the device is started.  We haven't yet considered how to change
> running devices, but I think the semantics we have since the respin of
> mdevctl separate saved config vs running devices in order to generalize
> the support of transient devices.

Yeah, my comment was aimed specifically at changes to a running device.
 When considering only the config file I agree: the new key value can
just replace the existing key value.