[ovirt-devel] Re: device compatibility interface for live migration with assigned devices

On Thu, 2020-09-10 at 14:38 +0200, Cornelia Huck wrote: > On Wed, 9 Sep 2020 10:13:09 +0800 > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > > > 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.) > > > > yes, it should work in some conditions. > > but it may not be that good in some cases when v5 and v4 in the name string > > of mdev type identify hardware generation (e.g. v4 for gen8, and v5 for > > gen9) > > > > e.g. > > (1). when src mdev type is v4 and target mdev type is v5 as > > software does not support it initially, and v4 and v5 identify hardware > > differences. > > My first hunch here is: Don't introduce types that may be compatible > later. Either make them compatible, or make them distinct by design, > and possibly add a different, compatible type later. > > > then after software upgrade, v5 is now compatible to v4, should the > > software now downgrade mdev type from v5 to v4? > > not sure if moving hardware generation info into a separate attribute > > from mdev type name is better. e.g. remove v4, v5 in mdev type, while use > > compatible_pci_ids to identify compatibility. > > If the generations are compatible, don't mention it in the mdev type. > If they aren't, use distinct types, so that management software doesn't > have to guess. At least that would be my naive approach here. yep that is what i would prefer to see too. > > > > > (2) name string of mdev type is composed by "driver_name + type_name". > > in some devices, e.g. qat, different generations of devices are binding to > > drivers of different names, e.g. "qat-v4", "qat-v5". > > then though type_name is equal, mdev type is not equal. e.g. > > "qat-v4-type1", "qat-v5-type1". > > I guess that shows a shortcoming of that "driver_name + type_name" > approach? Or maybe I'm just confused. yes i really dont like haveing the version in the mdev-type name i would stongly perfger just qat-type-1 wehere qat is just there as a way of namespacing. although symmetric-cryto, asymmetric-cryto and compression woudl be a better name then type-1, type-2, type-3 if that is what they would end up mapping too. e.g. qat-compression or qat-aes is a much better name then type-1 higher layers of software are unlikely to parse the mdev names but as a human looking at them its much eaiser to understand if the names are meaningful. the qat prefix i think is important however to make sure that your mdev-types dont colide with other vendeors mdev types. so i woudl encurage all vendors to prefix there mdev types with etiher the device name or the vendor.