Re: [PATCH v5 0/4] introduction of migration_version attribute for VFIO live migration

On Fri, 17 Apr 2020 05:52:02 -0400 Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > On Fri, Apr 17, 2020 at 04:44:50PM +0800, Cornelia Huck wrote: > > On Mon, 13 Apr 2020 01:52:01 -0400 > > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > > > This patchset introduces a migration_version attribute under sysfs of VFIO > > > Mediated devices. > > > > > > This migration_version attribute is used to check migration compatibility > > > between two mdev devices. > > > > > > Currently, it has two locations: > > > (1) under mdev_type node, > > > which can be used even before device creation, but only for mdev > > > devices of the same mdev type. > > > (2) under mdev device node, > > > which can only be used after the mdev devices are created, but the src > > > and target mdev devices are not necessarily be of the same mdev type > > > (The second location is newly added in v5, in order to keep consistent > > > with the migration_version node for migratable pass-though devices) > > > > What is the relationship between those two attributes? > > > (1) is for mdev devices specifically, and (2) is provided to keep the same > sysfs interface as with non-mdev cases. so (2) is for both mdev devices and > non-mdev devices. > > in future, if we enable vfio-pci vendor ops, (i.e. a non-mdev device > is binding to vfio-pci, but is able to register migration region and do > migration transactions from a vendor provided affiliate driver), > the vendor driver would export (2) directly, under device node. > It is not able to provide (1) as there're no mdev devices involved. Ok, creating an alternate attribute for non-mdev devices makes sense. However, wouldn't that rather be a case (3)? The change here only refers to mdev devices.

> > > Is existence (and compatibility) of (1) a pre-req for possible > > existence (and compatibility) of (2)? > > > no. (2) does not reply on (1). Hm. Non-existence of (1) seems to imply "this type does not support migration". If an mdev created for such a type suddenly does support migration, it feels a bit odd.

(It obviously cannot be a prereq for what I called (3) above.) > > > Does userspace need to check (1) or can it completely rely on (2), if > > it so chooses? > > > I think it can completely reply on (2) if compatibility check before > mdev creation is not required. > > > If devices with a different mdev type are indeed compatible, it seems > > userspace can only find out after the devices have actually been > > created, as (1) does not apply? > yes, I think so. How useful would it be for userspace to even look at (1) in that case? It only knows if things have a chance of working if it actually goes ahead and creates devices.

> > > One of my worries is that the existence of an attribute with the same > > name in two similar locations might lead to confusion. But maybe it > > isn't a problem. > > > Yes, I have the same feeling. but as (2) is for sysfs interface > consistency, to make it transparent to userspace tools like libvirt, > I guess the same name is necessary? What do we actually need here, I wonder? (1) and (2) seem to serve slightly different purposes, while (2) and what I called (3) have the same purpose. Is it important to userspace that (1) and (2) have the same name?