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> 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> 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?