device compatibility interface for live migration with assigned devices

[Cornelia Huck]

On Mon, 31 Aug 2020 12:43:44 +0800
Yan Zhao <yan.y.zhao at intel.com> wrote:

> On Fri, Aug 28, 2020 at 03:04:12PM +0100, Sean Mooney wrote:
> > On Fri, 2020-08-28 at 15:47 +0200, Cornelia Huck wrote:  
> > > On Wed, 26 Aug 2020 14:41:17 +0800
> > > Yan Zhao <yan.y.zhao at intel.com> wrote:
> > >   
> > > > previously, we want to regard the two mdevs created with dsa-1dwq x 30 and
> > > > dsa-2dwq x 15 as compatible, because the two mdevs consist equal resources.
> > > > 
> > > > But, as it's a burden to upper layer, we agree that if this condition
> > > > happens, we still treat the two as incompatible.
> > > > 
> > > > To fix it, either the driver should expose dsa-1dwq only, or the target
> > > > dsa-2dwq needs to be destroyed and reallocated via dsa-1dwq x 30.  
> > > 
> > > AFAIU, these are mdev types, aren't they? So, basically, any management
> > > software needs to take care to use the matching mdev type on the target
> > > system for device creation?  
> > 
> > or just do the simple thing of use the same mdev type on the source and dest.
> > matching mdevtypes is not nessiarly trivial. we could do that but we woudl have
> > to do that in python rather then sql so it would be slower to do at least today.
> > 
> > we dont currently have the ablity to say the resouce provider must have 1 of these
> > set of traits. just that we must have a specific trait. this is a feature we have
> > disucssed a couple of times and delayed untill we really really need it but its not out
> > of the question that we could add it for this usecase. i suspect however we would do exact
> > match first and explore this later after the inital mdev migration works.  
> 
> Yes, I think it's good.
> 
> still, I'd like to put it more explicitly to make ensure it's not missed:
> the reason we want to specify compatible_type as a trait and check
> whether target compatible_type is the superset of source
> compatible_type is for the consideration of backward compatibility.
> e.g.
> an old generation device may have a mdev type xxx-v4-yyy, while a newer
> generation  device may be of mdev type xxx-v5-yyy.
> with the compatible_type traits, the old generation device is still
> able to be regarded as compatible to newer generation device even their
> mdev types are not equal.

If you want to support migration from v4 to v5, can't the (presumably
newer) driver that supports v5 simply register the v4 type as well, so
that the mdev can be created as v4? (Just like QEMU versioned machine
types work.)