Re: [Qemu-devel] [libvirt] [RFC v2] libvirt vGPU QEMU integration

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 28 Sep 2016 12:22:35 -0700
Neo Jia <cjia@nvidia.com> wrote:

> On Thu, Sep 22, 2016 at 03:26:38PM +0100, Daniel P. Berrange wrote:
> > On Thu, Sep 22, 2016 at 08:19:21AM -0600, Alex Williamson wrote:  
> > > On Thu, 22 Sep 2016 09:41:20 +0530
> > > Kirti Wankhede <kwankhede@nvidia.com> wrote:
> > >   
> > > > >>>>> My concern is that a type id seems arbitrary but we're specifying that
> > > > >>>>> it be unique.  We already have something unique, the name.  So why try
> > > > >>>>> to make the type id unique as well?  A vendor can accidentally create
> > > > >>>>> their vendor driver so that a given name means something very
> > > > >>>>> specific.  On the other hand they need to be extremely deliberate to
> > > > >>>>> coordinate that a type id means a unique thing across all their product
> > > > >>>>> lines.
> > > > >>>>>         
> > > > >>>>
> > > > >>>> Let me clarify, type id should be unique in the list of
> > > > >>>> mdev_supported_types. You can't have 2 directories in with same name.      
> > > > >>>
> > > > >>> Of course, but does that mean it's only unique to the machine I'm
> > > > >>> currently running on?  Let's say I have a Tesla P100 on my system and
> > > > >>> type-id 11 is named "GRID-M60-0B".  At some point in the future I
> > > > >>> replace the Tesla P100 with a Q1000 (made up).  Is type-id 11 on that
> > > > >>> new card still going to be a "GRID-M60-0B"?  If not then we've based
> > > > >>> our XML on the wrong attribute.  If the new device does not support
> > > > >>> "GRID-M60-0B" then we should generate an error, not simply initialize
> > > > >>> whatever type-id 11 happens to be on this new card.
> > > > >>>       
> > > > >>
> > > > >> If there are 2 M60 in the system then you would find '11' type directory
> > > > >> in mdev_supported_types of both M60. If you have P100, '11' type would
> > > > >> not be there in its mdev_supported_types, it will have different types.
> > > > >>
> > > > >> For example, if you replace M60 with P100, but XML is not updated. XML
> > > > >> have type '11'. When libvirt would try to create mdev device, libvirt
> > > > >> would have to find 'create' file in sysfs in following directory format:
> > > > >>
> > > > >>  --- mdev_supported_types
> > > > >>      |-- 11
> > > > >>      |   |-- create
> > > > >>
> > > > >> but now for P100, '11' directory is not there, so libvirt should throw
> > > > >> error on not able to find '11' directory.    
> > > > > 
> > > > > This really seems like an accident waiting to happen.  What happens
> > > > > when the user replaces their M60 with an Intel XYZ device that happens
> > > > > to expose a type 11 mdev class gpu device?  How is libvirt supposed to
> > > > > know that the XML used to refer to a GRID-M60-0B and now it's an
> > > > > INTEL-IGD-XYZ?  Doesn't basing the XML entry on the name and removing
> > > > > yet another arbitrary requirement that we have some sort of globally
> > > > > unique type-id database make a lot of sense?  The same issue applies
> > > > > for simple debug-ability, if I'm reviewing the XML for a domain and the
> > > > > name is the primary index for the mdev device, I know what it is.
> > > > > Seeing type-id='11' is meaningless.
> > > > >    
> > > > 
> > > > Let me clarify again, type '11' is a string that vendor driver would
> > > > define (see my previous reply below) it could be "11" or "GRID-M60-0B".
> > > > If 2 vendors used same string we can't control that. right?
> > > > 
> > > >   
> > > > >>>> Lets remove 'id' from type id in XML if that is the concern. Supported
> > > > >>>> types is going to be defined by vendor driver, so let vendor driver
> > > > >>>> decide what to use for directory name and same should be used in device
> > > > >>>> xml file, it could be '11' or "GRID M60-0B":
> > > > >>>>
> > > > >>>>     <device>
> > > > >>>>       <name>my-vgpu</name>
> > > > >>>>       <parent>pci_0000_86_00_0</parent>
> > > > >>>>       <capability type='mdev'>
> > > > >>>>         <type='11'/>
> > > > >>>>         ...
> > > > >>>>       </capability>
> > > > >>>>     </device>  
> > > 
> > > Then let's get rid of the 'name' attribute and let the sysfs directory
> > > simply be the name.  Then we can get rid of 'type' altogether so we
> > > don't have this '11' vs 'GRID-M60-0B' issue.  Thanks,  
> > 
> > That sounds nice to me - we don't need two unique identifiers if
> > one will do.  
> 
> Hi Alex and Daniel,
> 
> I just had some internal discussions here within NVIDIA and found out that
> actually the name/label potentially might not be unique and the "id" will be. 
> So I think we still would like to keep both so the id is the programmatic id
> and the name/label is a human readable string for it, which might get changed to
> be non-unique by outside of engineering.

I think your discovery only means that for your vendor driver, the name
will be "11" (as a string).  Perhaps you'd like some sort of vendor
provided description within each type, but I am not in favor of having
an arbitrary integer value imply something specific within the sysfs
interface.  IOW, the NVIDIA vendor driver should be able to create:

11
├── create
├── description
├── etc
└── resolution

While Intel might create:

Skylake-vGPU
├── create
├── description
├── etc
└── resolution

Maybe "description" is optional for vendors that use useful names?
Thanks,

Alex