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    SubjectRe: [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@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@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?

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