Re: device compatibility interface for live migration with assigned devices

[Sean Mooney]

On Thu, 2020-08-20 at 08:39 +0800, Yan Zhao wrote:
> On Tue, Aug 18, 2020 at 11:36:52AM +0200, Cornelia Huck wrote:
> > On Tue, 18 Aug 2020 10:16:28 +0100
> > Daniel P. Berrangé <berrange@redhat.com> wrote:
> > 
> > > On Tue, Aug 18, 2020 at 05:01:51PM +0800, Jason Wang wrote:
> > > >    On 2020/8/18 下午4:55, Daniel P. Berrangé wrote:
> > > > 
> > > >  On Tue, Aug 18, 2020 at 11:24:30AM +0800, Jason Wang wrote:
> > > > 
> > > >  On 2020/8/14 下午1:16, Yan Zhao wrote:
> > > > 
> > > >  On Thu, Aug 13, 2020 at 12:24:50PM +0800, Jason Wang wrote:
> > > > 
> > > >  On 2020/8/10 下午3:46, Yan Zhao wrote:  
> > > >  we actually can also retrieve the same information through sysfs, .e.g
> > > > 
> > > >  |- [path to device]
> > > >     |--- migration
> > > >     |     |--- self
> > > >     |     |   |---device_api
> > > >     |    |   |---mdev_type
> > > >     |    |   |---software_version
> > > >     |    |   |---device_id
> > > >     |    |   |---aggregator
> > > >     |     |--- compatible
> > > >     |     |   |---device_api
> > > >     |    |   |---mdev_type
> > > >     |    |   |---software_version
> > > >     |    |   |---device_id
> > > >     |    |   |---aggregator
> > > > 
> > > > 
> > > >  Yes but:
> > > > 
> > > >  - You need one file per attribute (one syscall for one attribute)
> > > >  - Attribute is coupled with kobject
> > 
> > Is that really that bad? You have the device with an embedded kobject
> > anyway, and you can just put things into an attribute group?
> > 
> > [Also, I think that self/compatible split in the example makes things
> > needlessly complex. Shouldn't semantic versioning and matching already
> > cover nearly everything? I would expect very few cases that are more
> > complex than that. Maybe the aggregation stuff, but I don't think we
> > need that self/compatible split for that, either.]
> 
> Hi Cornelia,
> 
> The reason I want to declare compatible list of attributes is that
> sometimes it's not a simple 1:1 matching of source attributes and target 
> attributes
> as I demonstrated below,
> source mdev of (mdev_type i915-GVTg_V5_2 + aggregator 1) is compatible to
> target mdev of (mdev_type i915-GVTg_V5_4 + aggregator 2),
>                (mdev_type i915-GVTg_V5_8 + aggregator 4)
the way you are doing the nameing is till really confusing by the way
if this has not already been merged in the kernel can you chagne the mdev
so that mdev_type i915-GVTg_V5_2 is 2 of mdev_type i915-GVTg_V5_1 instead of 
half the device

currently you need to deived the aggratod by the number at the end of the mdev 
type to figure out
how much of the phsicial device is being used with is a very unfridly api 
convention

the way aggrator are being proposed in general is not really someting i like 
but i thin this at least
is something that should be able to correct.

with the complexity in the mdev type name + aggrator i suspect that this will 
never be support
in openstack nova directly requireing integration via cyborg unless we can pre 
partion the
device in to mdevs staicaly and just ignore this.

this is way to vendor sepecif to integrate into something like openstack in 
nova unless we can guarentee
taht how aggreator work will be portable across vendors genericly.

> 
> and aggragator may be just one of such examples that 1:1 matching does not
> fit.
for openstack nova i dont see us support anything beyond the 1:1 case where the 
mdev type does not change.

i woudl really prefer if there was just one mdev type that repsented the 
minimal allcatable unit and the
aggragaotr where used to create compostions of that. i.e instad of 
i915-GVTg_V5_2 beign half the device,
have 1 mdev type i915-GVTg and if the device support 8 of them then we can 
aggrate 4 of i915-GVTg

if you want to have muplie mdev type to model the different amoutn of the 
resouce e.g. i915-GVTg_small i915-GVTg_large
that is totlaly fine too or even i915-GVTg_4 indcating it sis 4 of i915-GVTg

failing that i would just expose an mdev type per composable resouce and allow 
us to compose them a the user level with
some other construct mudeling a attament to the device. e.g. create composed 
mdev or somethig that is an aggreateion of
multiple sub resouces each of which is an mdev. so kind of like how bond port 
work. we would create an mdev for each of
the sub resouces and then create a bond or aggrated mdev by reference the other 
mdevs by uuid then attach only the
aggreated mdev to the instance.

the current aggrator syntax and sematic however make me rather uncofrotable 
when i think about orchestating vms on top
of it even to boot them let alone migrate them.
> 
> So, we explicitly list out self/compatible attributes, and management
> tools only need to check if self attributes is contained compatible
> attributes.
> 
> or do you mean only compatible list is enough, and the management tools
> need to find out self list by themselves?
> But I think provide a self list is easier for management tools.
> 
> Thanks
> Yan
>