Re: [PATCH v2 1/2] vfio/mdev: add version attribute for mdev device

From: Alex Williamson
Date: Thu May 09 2019 - 00:06:40 EST

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On Wed, 8 May 2019 23:10:55 -0400
Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:

> On Thu, May 09, 2019 at 05:22:42AM +0800, Alex Williamson wrote:
> > On Wed, 8 May 2019 07:27:40 -0400
> > Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
> >
> > > On Wed, May 08, 2019 at 05:18:26AM +0800, Alex Williamson wrote:
> > > > On Sun, 5 May 2019 21:49:04 -0400
> > > > Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
> > > >
> > > > > version attribute is used to check two mdev devices' compatibility.
> > > > >
> > > > > The key point of this version attribute is that it's rw.
> > > > > User space has no need to understand internal of device version and no
> > > > > need to compare versions by itself.
> > > > > Compared to reading version strings from both two mdev devices being
> > > > > checked, user space only reads from one mdev device's version attribute.
> > > > > After getting its version string, user space writes this string into the
> > > > > other mdev device's version attribute. Vendor driver of mdev device
> > > > > whose version attribute being written will check device compatibility of
> > > > > the two mdev devices for user space and return success for compatibility
> > > > > or errno for incompatibility.
> > > > > So two readings of version attributes + checking in user space are now
> > > > > changed to one reading + one writing of version attributes + checking in
> > > > > vendor driver.
> > > > > Format and length of version strings are now private to vendor driver
> > > > > who can define them freely.
> > > > >
> > > > > __ user space
> > > > > /\ \
> > > > > / \write
> > > > > / read \
> > > > > ______/__ ___\|/___
> > > > > | version | | version |-->check compatibility
> > > > > ----------- -----------
> > > > > mdev device A mdev device B
> > > > >
> > > > > This version attribute is optional. If a mdev device does not provide
> > > > > with a version attribute, this mdev device is incompatible to all other
> > > > > mdev devices.
> > > > >
> > > > > Live migration is able to take advantage of this version attribute.
> > > > > Before user space actually starts live migration, it can first check
> > > > > whether two mdev devices are compatible.
> > > > >
> > > > > v2:
> > > > > 1. added detailed intent and usage
> > > > > 2. made definition of version string completely private to vendor driver
> > > > > (Alex Williamson)
> > > > > 3. abandoned changes to sample mdev drivers (Alex Williamson)
> > > > > 4. mandatory --> optional (Cornelia Huck)
> > > > > 5. added description for errno (Cornelia Huck)
> > > > >
> > > > > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > > > Cc: Erik Skultety <eskultet@xxxxxxxxxx>
> > > > > Cc: "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx>
> > > > > Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
> > > > > Cc: "Tian, Kevin" <kevin.tian@xxxxxxxxx>
> > > > > Cc: Zhenyu Wang <zhenyuw@xxxxxxxxxxxxxxx>
> > > > > Cc: "Wang, Zhi A" <zhi.a.wang@xxxxxxxxx>
> > > > > Cc: Neo Jia <cjia@xxxxxxxxxx>
> > > > > Cc: Kirti Wankhede <kwankhede@xxxxxxxxxx>
> > > > > Cc: Daniel P. BerrangÃ <berrange@xxxxxxxxxx>
> > > > > Cc: Christophe de Dinechin <dinechin@xxxxxxxxxx>
> > > > >
> > > > > Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > > > > ---
> > > > > Documentation/vfio-mediated-device.txt | 140 +++++++++++++++++++++++++
> > > > > 1 file changed, 140 insertions(+)
> > > > >
> > > > > diff --git a/Documentation/vfio-mediated-device.txt b/Documentation/vfio-mediated-device.txt
> > > > > index c3f69bcaf96e..013a764968eb 100644
> > > > > --- a/Documentation/vfio-mediated-device.txt
> > > > > +++ b/Documentation/vfio-mediated-device.txt
> > > > > @@ -202,6 +202,7 @@ Directories and files under the sysfs for Each Physical Device
> > > > > | | |--- available_instances
> > > > > | | |--- device_api
> > > > > | | |--- description
> > > > > + | | |--- version
> > > > > | | |--- [devices]
> > > > > | |--- [<type-id>]
> > > > > | | |--- create
> > > > > @@ -209,6 +210,7 @@ Directories and files under the sysfs for Each Physical Device
> > > > > | | |--- available_instances
> > > > > | | |--- device_api
> > > > > | | |--- description
> > > > > + | | |--- version
> > > > > | | |--- [devices]
> > > > > | |--- [<type-id>]
> > > > > | |--- create
> > > > > @@ -216,6 +218,7 @@ Directories and files under the sysfs for Each Physical Device
> > > > > | |--- available_instances
> > > > > | |--- device_api
> > > > > | |--- description
> > > > > + | |--- version
> > > > > | |--- [devices]
> > > >
> > > > I thought there was a request to make this more specific to migration
> > > > by renaming it to something like migration_version. Also, as an
> > > >
> > > so this attribute may not only include a mdev device's parent device info and
> > > mdev type, but also include numeric software version of vendor specific
> > > migration code, right?
> >
> > It's a vendor defined string, it should be considered opaque to the
> > user, the vendor can include whatever they feel is relevant.
> >
> > > This actually makes sense.
> > > So, do I need to add a disclaimer in this doc like:
> > > vendor driver should be responsible by itself for a mdev device's migration
> > > compatibility.
> >
> > I thought that was the purpose of this attribute.
> >
> > > During migration setup phase, general migration code in user space VFIO only
> > > checks this version of VFIO migration region, and will not check software version
> > > of vendor specific migration code.
> >
> > What is "software version of vendor specific migration code"? If
> > you're asking whether anything will check for parent device
> > compatibility or parent vendor driver compatibility, the answer is no,
> > that's what this interface is meant to provide. Userspace should need
> > to do nothing more than verify the mdev types match and then use the
> > version attribute to confirm source to target compatibility.
> >
> > > It is suggested to incorporate at least parent device info and software version
> > > of vendor specific migration code into this migration_version attribute.
> >
> > We can make recommendations as "best practices", but ultimately it's an
> > opaque string defined by the vendor driver.
> >
> > But you never addressed my comment that previous reviews asked for the
> > attribute to be named something more specific to migration.
> >
> I aggree to rename it to migration_version.
>
> > > > optional attribute, it seems the example should perhaps not add it to
> > > > all types to illustrate that it is not required.
> > > ok. got it.
> > >
> > >
> > > > >
> > > > > * [mdev_supported_types]
> > > > > @@ -246,6 +249,143 @@ Directories and files under the sysfs for Each Physical Device
> > > > > This attribute should show the number of devices of type <type-id> that can be
> > > > > created.
> > > > >
> > > > > +* version
> > > > > +
> > > > > + This attribute is rw, and is optional.
> > > > > + It is used to check device compatibility between two mdev devices and is
> > > >
> > > > between two mdev devices of the same type.
> > > >
> > > ok. got it.
> > > But I have a question about aggregation proposed earlier.
> > > Do we also have to assume the two mdev devices are of the same aggregation
> > > count?
> > > However, aggregation count is not available before a mdev device is created. :(
> >
> > We don't support aggregation yet, but yes, that's going to introduce
> > issues here. Any configuration beyond the base mdev type would imply
> > that the base type could be compatible for migration, but the specific
> > instances might not. Resolving that would imply that our version
> > information needs to be relative to an instance, not just the base type.
> >
> > How would we extend this interface to support that? We could have a
> > version attribute on each device instance, which might report something
> > like:
> >
> > 0123456789,aggregate=2
> >
> > IOW the device instance of version concatenates the mdev type version
> > with the additional create parameters for that device. Writing this to
> > the type attribute should be parsed by the vendor driver as support for
> > given base device with specified additional create parameters.
> >
> > I'm afraid this also bring us around to treacherous questions around
> > who is responsible for creating that device on the migration target and
> > where is this meta information about the device exposed. Maybe instead
> > of a per instance version attribute we would instead expose the create
> > parameters as an attribute per instance and it would be userspace's
> > responsibility to create a version string from the mdev type and create
> > parameters similar to above. This would also make it possible to
> > create a compatible instance on the target without pre-knowledge of how
> > the device was created.
> >
> > Also, this issue exists already, but compatibility and capacity are two
> > separate things, I think we want to limit this interface to the
> > former. For instance, if I want to migrate an i915-GVTg_V5_1 device to
> > another system where available_instances for that type is zero, the
> > version attribute should strictly report the device compatibility, it's
> > not responsible for returning an errno due to lack of resources.
> > Similarly if we were to do something with aggregation, the version
> > attribute would strictly report if the target supports creating that
> > device with those parameters, not whether it has capacity to create
> > such as device at that instant in time.
> >
> I think it's good to have a migration_version attribute under each device
> instance.
> It has two pros:
> 1. vendor driver can incorperate into the string things like:
> parent device info + mdev type + aggregate count + software version

The only thing added here is aggregate count, the rest is available for
the base type.

> 2. even for non mdev devices, like a VF in SRIOV
> PF driver can export this migration_version under a VF instance. so a VF is
> possible to migrate with vfio-pci driver installed. (though with current VFIO
> live migration RFCs, with vfio-pci driver is not able to migrate. but it
> provides a possibility)

I don't follow here, migration version is only one piece of the puzzle
to enable the migration of a device, if a PF driver wants to make a VF
assignable and migratable, it can wrap it in an mdev layer and export
it within the infrastructure we're developing.

> could we maintain two migration_version attributes?
> "create parameter + mdev_type migraton_version" for user space software to
> create a migration compatible mdev device.
> "per device instance migration_version" for verifying migration compatibility of
> devices already created.

I don't think so. If it wasn't obvious in my stream of consciousness
previous reply, I prefer the latter mechanism where an instance exposes
the creation attributes allowing the user to concatenate the version
string and creation attributes to ask the type level version about
compatibility. I think it's confusing to have a version on the
instance that reports something different than the version on the type
but we cannot remove the version on the type because we need to be able
to test compatibility before instantiating an instance. Therefore
let's not put a version on the instance is the conclusion I've come
to. Thanks,

Alex

> > > > > + accessed in pairs between the two mdev devices being checked.
> > > >
> > > > "in pairs"?
> > > I meant, user space needs to access version attributes from two mdev device.
> > > but seems that it's needless to mention that... I'll remove it :)
> > >
> > >
> > > > > + The intent of this attribute is to make an mdev device's version opaque to
> > > > > + user space, so instead of reading two mdev devices' version strings and
> > > >
> > > > perhaps "...instead of reading the version string of two mdev devices
> > > > and comparing them in userspace..."
> > > yes, better, thanks:)
> > >
> > > > > + comparing in userspace, user space should only read one mdev device's version
> > > > > + attribute, and writes this version string into the other mdev device's version
> > > > > + attribute. Then vendor driver of mdev device whose version attribute being
> > > > > + written would check the incoming version string and tell user space whether
> > > > > + the two mdev devices are compatible via return value. That's why this
> > > > > + attribute is writable.
> > > > > +
> > > > > + when reading this attribute, it should show device version string of
> > > > > + the device of type <type-id>.
> > > > > +
> > > > > + This string is private to vendor driver itself. Vendor driver is able to
> > > > > + freely define format and length of device version string.
> > > > > + e.g. It can use a combination of pciid of parent device + mdev type.
> > > >
> > > > Can the user assume the data contents of the string is ascii
> > > > characters? It's good that the vendor driver defines the format and
> > > > length, but the user probably needs some expectation bounding that
> > > > length. Should we define it as no larger than PATH_MAX (4096), or maybe
> > > > NAME_MAX (255) might be more reasonable?
> > > I think so. I'll add those restrictions in next revision.
> >
> > If we start adding creation parameters, PATH_MAX may actually be the
> > more reasonable limit.
> >
> got it.
> > > > > +
> > > > > + When writing a string to this attribute, vendor driver should analyze this
> > > > > + string and check whether the mdev device being identified by this string is
> > > > > + compatible with the mdev device for this attribute. vendor driver should then
> > > >
> > > > Compatible for what purpose? I think this is where specifically
> > > > calling this a migration_version potentially has value.
> > > yes. if it also covers version of vendor specific migration code, calling it
> > > migration_version is more appropriate.
> >
> > I think we're discussing an interface that validates "I [the vendor
> > driver] am able to import the state of this version", therefore it must
> > include every relevant aspect of the vendor support for that.
> >
> > > > > + return written string's length if it regards the two mdev devices are
> > > > > + compatible; vendor driver should return negative errno if it regards the two
> > > > > + mdev devices are not compatible.
> > > >
> > > > IOW, the write(2) will succeed if the version is determined to be
> > > > compatible and otherwise fail with vendor specific errno.
> > > >
> > > thanks:)
> > >
> > > > > +
> > > > > + User space should treat ANY of below conditions as two mdev devices not
> > > > > + compatible:
> > > >
> > > > (0) The mdev devices are not of the same type.
> > > >
> > > the same as above. do we also need to take aggregation count into consideration?
> > >
> > > > > + (1) any one of the two mdev devices does not have a version attribute
> > > > > + (2) error when read from one mdev device's version attribute
> > > >
> > > > Is this intended to support that the vendor driver can supply a version
> > > > attribute but not support migration? TBH, this sounds like a vendor
> > > > driver bug, but maybe it's necessary if the vendor driver could have
> > > > some types that support migration and others that do not? IOW, we're
> > > > supplying the same attribute groups to all devices from a vendor, in
> > > > which case my comment above regarding an example type without a version
> > > > attribute might be invalid.
> > > hmm, this is to make life easier for vendor driver to have some types that
> > > support migration and others that do not. while we can get rid of returning
> > > errno by providing different attribute groups to different devices, the way of
> > > returning errno gives a simpler choice to vendors.
> >
> > Yes, I think it might be overly complicated to provide different
> > attribute groups for different devices, we have more flexibility if the
> > user does not make any assumptions based only on the presence of a
> > version attribute.
> >
> > > > > + (3) error when write one mdev device's version string to the other mdev
> > > > > + device's version attribute
> > > > > +
> > > > > + User space should regard two mdev devices compatible when ALL of below
> > > > > + conditions are met:
> > > >
> > > > (0) The mdev devices are of the same type
> > > >
> > > > > + (1) success when read from one mdev device's version attribute.
> > > > > + (2) success when write one mdev device's version string to the other mdev
> > > > > + device's version attribute
> > > > > +
> > > > > + Errno:
> > > > > + If vendor driver wants to claim a mdev device incompatible to all other mdev
> > > > > + devices, it should not register version attribute for this mdev device. But if
> > > > > + a vendor driver has already registered version attribute and it wants to claim
> > > > > + a mdev device incompatible to all other mdev devices, it needs to return
> > > > > + -ENODEV on access to this mdev device's version attribute.
> > > > > + If a mdev device is only incompatible to certain mdev devices, write of
> > > > > + incompatible mdev devices's version strings to its version attribute should
> > > > > + return -EINVAL;
> > > >
> > > > I think it's best not to define the specific errno returned for a
> > > > specific situation, let the vendor driver decide, userspace simply
> > > > needs to know that an errno on read indicates the device does not
> > > > support migration version comparison and that an errno on write
> > > > indicates the devices are incompatible or the target doesn't support
> > > > migration versions.
> > > >
> > > yes, user space only gets 0 or 1 as return code, not those errno.
> > > maybe I only need to describe errno in patch 2/2.
> > >
> > > > > +
> > > > > + This attribute can be taken advantage of by live migration.
> > > > > + If user space detects two mdev devices are compatible through version
> > > > > + attribute, it can start migration between the two mdev devices, otherwise it
> > > > > + should abort its migration attempts between the two mdev devices.
> > > > > +
> > > > > + Example Usage:
> > > > > + case 1:
> > > > > + source side mdev device is of uuid 5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd,
> > > > > + its mdev type is i915-GVTg_V5_4. pci id of parent device is 8086-193b.
> > > > > + target side mdev device is if of uuid 882cc4da-dede-11e7-9180-078a62063ab1,
> > > > > + its mdev type is i915-GVTg_V5_4. pci id of parent device is 8086-193b.
> > > > > +
> > > > > + # readlink /sys/bus/pci/devices/0000\:00\:02.0/\
> > > > > + 5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd/mdev_type
> > > > > + ../mdev_supported_types/i915-GVTg_V5_4
> > > > > +
> > > > > + # readlink /sys/bus/pci/devices/0000\:00\:02.0/\
> > > > > + 882cc4da-dede-11e7-9180-078a62063ab1/mdev_type
> > > > > + ../mdev_supported_types/i915-GVTg_V5_4
> > > > > +
> > > > > + (1) read source side mdev device's version.
> > > > > + #cat \
> > > > > + /sys/bus/pci/devices/0000\:00\:02.0/5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd/\
> > > > > + mdev_type/version
> > > > > + 8086-193b-i915-GVTg_V5_4
> > > >
> > > > Is this really the version information exposed in 2/2? This is opaque,
> > > > so of course you can add things later, but it seems short sighted not
> > > > to even append a version 0 tag to account for software compatibility
> > > > differences since the above only represents a parent and mdev type
> > > > based version.
> > > >
> > > yes, currently in 2/2, the version only includes <vendor id> + <device id> +
> > > <mdev type>. but you are right, it's better to include software migration
> > > version number.
> > > so vendor drivers have below 3 ways to designate a mdev device has no migration
> > > capability.
> > > 1. not registering migration_version attribute
> > > 2. on reading migration_version, returning errno
> > > 3. on reading migration_version, returning string indicating non-migratable.
> > >
> > > The reason of not giving up way 2 is that maybe it can accelerate user space
> > > getting information of device incompatible. if we only keep way 3, it would not
> > > know this info until writing this string to target attribute.
> > >
> > > do you agree?
> >
> > The string is opaque to the user, so if you're asking in (3) that the
> > user read and parse some information in the string that indicates the
> > device is non-migratable then no, I don't agree with that policy. If
> > reading the version attribute produces a result, the only thing the
> > user can do with that result is to test it by writing it to another
> > version attribute. Thanks,
> >
> > Alex
>
> ok. so we will keep way 2 as a valid choice :)
>
> Thanks
> Yan
> > > > > + (2) write source side mdev device's version string into target side mdev
> > > > > + device's version attribute.
> > > > > + # echo 8086-193b-i915-GVTg_V5_4 >
> > > > > + /sys/bus/pci/devices/0000\:00\:02.0/882cc4da-dede-11e7-9180-078a62063ab1/\
> > > > > + mdev_type/version
> > > > > + # echo $?
> > > > > + 0
> > > >
> > > > TBH, there's a lot of superfluous information in this example that can
> > > > be stripped out. For example:
> > > >
> > > > "
> > > > (1) Compare mdev types:
> > > >
> > > > The mdev type of an instantiated device can be read from the mdev_type
> > > > link within the device instance in sysfs, for example:
> > > >
> > > > # basename $(readlink -f /sys/bus/mdev/devices/$MDEV_UUID/mdev_type/)
> > > >
> > > > The mdev types available on a given host system can also be found
> > > > through /sys/class/mdev_bus, for example:
> > > >
> > > > # ls /sys/class/mdev_bus/*/mdev_supported_types/
> > > >
> > > > Migration is only possible between devices of the same mdev type.
> > > >
> > > > (2) Retrieve the mdev source version:
> > > >
> > > > The migration version information can either be read from the mdev_type
> > > > link on an instantiated device:
> > > >
> > > > # cat /sys/bus/mdev/devices/$UUID1/mdev_type/version
> > > >
> > > > Or it can be read from the mdev type definition, for example:
> > > >
> > > > # cat /sys/class/mdev_bus/*/mdev_supported_types/$MDEV_TYPE/version
> > > >
> > > > If reading the source version generates an error, migration is not
> > > > possible. NB, there might be several parent devices for a given mdev
> > > > type on a host system, each may support or expose different versions.
> > > > Matching the specific mdev type to a parent may become important in
> > > > such configurations.
> > > >
> > > > (3) Test source version at target:
> > > >
> > > > Given a version as outlined above, its compatibility to an instantiated
> > > > device of the same mdev type can be tested as:
> > > >
> > > > # echo $VERSION > /sys/bus/mdev/devices/$UUID2/mdev_type/version
> > > >
> > > > If this write fails, the source and target versions are not compatible
> > > > or the target does not support migration.
> > > >
> > > > Compatibility can also be tested prior to target device creation using
> > > > the mdev type definition for a parent device with a previously found
> > > > matching mdev type, for example:
> > > >
> > > > # echo $VERSION > /sys/class/mdev_bus/$PARENT/mdev_supported_types/$MDEV_TYPE/version
> > > >
> > > > Again, an error writing the version indicates that an instance of this
> > > > mdev type would not support a migration from the provided version.
> > > > "
> > > >
> > > > In particular from the provided example, the specific UUIDs, mdev
> > > > types, parent information, and contents of the version attribute do not
> > > > contribute to illustrating the protocol. In fact, displaying the
> > > > contents of the version attribute may tempt users to do comparison on
> > > > their own, especially given how easy it is to decide the GVT-g version
> > > > string.
> > > >
> > > got it!
> > > great thanks!
> > > I'll update it to the next revision.
> > > >
> > > > > +
> > > > > + in this case, user space's write to target side mdev device's version
> > > > > + attribute returns success to indicate the two mdev devices are compatible.
> > > > > +
> > > > > + case 2:
> > > > > + source side mdev device is of uuid 5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd,
> > > > > + its mdev type is i915-GVTg_V5_4. pci id of parent device is 8086-193b.
> > > > > + target side mdev device is if of uuid 882cc4da-dede-11e7-9180-078a62063ab1,
> > > > > + its mdev type is i915-GVTg_V5_4. pci id of parent device is 8086-191b.
> > > > > +
> > > > > + # readlink /sys/bus/pci/devices/0000\:00\:02.0/\
> > > > > + 5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd/mdev_type
> > > > > + ../mdev_supported_types/i915-GVTg_V5_4
> > > > > +
> > > > > + # readlink /sys/bus/pci/devices/0000\:00\:02.0/\
> > > > > + 882cc4da-dede-11e7-9180-078a62063ab1/mdev_type
> > > > > + ../mdev_supported_types/i915-GVTg_V5_4
> > > > > +
> > > > > + (1) read source side mdev device's version.
> > > > > + #cat \
> > > > > + /sys/bus/pci/devices/0000\:00\:02.0/5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd/\
> > > > > + mdev_type/version
> > > > > + 8086-193b-i915-GVTg_V5_4
> > > > > +
> > > > > + (2) write source side mdev device's version string into target side mdev
> > > > > + device's version attribute.
> > > > > + # echo 8086-193b-i915-GVTg_V5_4 >
> > > > > + /sys/bus/pci/devices/0000\:00\:02.0/882cc4da-dede-11e7-9180-078a62063ab1/\
> > > > > + mdev_type/version
> > > > > + -bash: echo: write error: Invalid argument
> > > > > +
> > > > > + in this case, user space's write to target side mdev device's version
> > > > > + attribute returns error to indicate the two mdev devices are incompatible.
> > > > > + (incompatible because pci ids of the two mdev devices' parent devices are
> > > > > + different).
> > > > > +
> > > > > + case 3:
> > > > > + source side mdev device is of uuid 5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd,
> > > > > + its mdev type is i915-GVTg_V5_4. pci id of parent device is 8086-193b.
> > > > > + But vendor driver does not provide version attribute for this device.
> > > > > +
> > > > > + (1) read source side mdev device's version.
> > > > > + #cat \
> > > > > + /sys/bus/pci/devices/0000\:00\:02.0/5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd/\
> > > > > + mdev_type/version
> > > > > + cat: '/sys/bus/pci/devices/0000:00:02.0/5ac1fb20-2bbf-4842-bb7e-36c58c3be9cd/\
> > > > > + mdev_type/version': No such file or directory
> > > > > +
> > > > > + in this case, user space reads source side mdev device's version attribute
> > > > > + which does not exist however. user space regards the two mdev devices as not
> > > > > + compatible and will not start migration between the two mdev devices.
> > > > > +
> > > > > +
> > > >
> > > > This is far too long for description and examples, it's not this
> > > > complicated. Thanks,
> > > >
> > > got it. I'll follow your above example :)
> > >
> > > thanks
> > > Yan
> > > > > * [device]
> > > > >
> > > > > This directory contains links to the devices of type <type-id> that have been
> > > >
> >

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