[PATCH v5 1/4] vfio/mdev: add migration_version attribute for mdev (under mdev_type node)
Yan Zhao
yan.y.zhao at intel.com
Wed Apr 15 08:58:28 UTC 2020
On Wed, Apr 15, 2020 at 03:28:51PM +0800, Erik Skultety wrote:
> On Mon, Apr 13, 2020 at 01:54:03AM -0400, Yan Zhao wrote:
> > migration_version attribute is used to check migration compatibility
> > between two mdev devices of the same mdev type.
> > The key is that it's rw and its data is opaque to userspace.
> >
> > Userspace reads migration_version of mdev device at source side and
> > writes the value to migration_version attribute of mdev device at target
> > side. It judges migration compatibility according to whether the read
> > and write operations succeed or fail.
> >
> > Currently, it is able to read/write migration_version attribute under two
> > places:
> >
> > (1) under mdev_type node
> > userspace is able to know whether two mdev devices are compatible before
> > a mdev device is created.
> >
> > userspace also needs to check whether the two mdev devices are of the same
> > mdev type before checking the migration_version attribute. It also needs
> > to check device creation parameters if aggregation is supported in future.
> >
> > (2) under mdev device node
> > userspace is able to know whether two mdev devices are compatible after
> > they are all created. But it does not need to check mdev type and device
> > creation parameter for aggregation as device vendor driver would have
> > incorporated those information into the migration_version attribute.
> >
> > __ userspace
> > /\ \
> > / \write
> > / read \
> > ________/__________ ___\|/_____________
> > | migration_version | | migration_version |-->check migration
> > --------------------- --------------------- compatibility
> > mdev device A mdev device B
> >
> > This patch is for mdev documentation about the first place (under
> > mdev_type node)
> >
> > Cc: Alex Williamson <alex.williamson at redhat.com>
> > Cc: Erik Skultety <eskultet at redhat.com>
> > Cc: "Dr. David Alan Gilbert" <dgilbert at redhat.com>
> > Cc: Cornelia Huck <cohuck at redhat.com>
> > Cc: "Tian, Kevin" <kevin.tian at intel.com>
> > Cc: Zhenyu Wang <zhenyuw at linux.intel.com>
> > Cc: "Wang, Zhi A" <zhi.a.wang at intel.com>
> > Cc: Neo Jia <cjia at nvidia.com>
> > Cc: Kirti Wankhede <kwankhede at nvidia.com>
> > Cc: Daniel P. Berrangé <berrange at redhat.com>
> > Cc: Christophe de Dinechin <dinechin at redhat.com>
> >
> > Reviewed-by: Cornelia Huck <cohuck at redhat.com>
> > Signed-off-by: Yan Zhao <yan.y.zhao at intel.com>
> >
> > ---
> > v5:
> > updated commit message a little to indicate this patch is for
> > migration_version attribute under mdev_type node
> >
> > v4:
> > fixed a typo. (Cornelia Huck)
> >
> > v3:
> > 1. renamed version to migration_version
> > (Christophe de Dinechin, Cornelia Huck, Alex Williamson)
> > 2. let errno to be freely defined by vendor driver
> > (Alex Williamson, Erik Skultety, Cornelia Huck, Dr. David Alan Gilbert)
> > 3. let checking mdev_type be prerequisite of migration compatibility
> > check. (Alex Williamson)
> > 4. reworded example usage section.
> > (most of this section came from Alex Williamson)
> > 5. reworded attribute intention section (Cornelia Huck)
> >
> > 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)
> > ---
> > .../driver-api/vfio-mediated-device.rst | 113 ++++++++++++++++++
> > 1 file changed, 113 insertions(+)
> >
> > diff --git a/Documentation/driver-api/vfio-mediated-device.rst b/Documentation/driver-api/vfio-mediated-device.rst
> > index 25eb7d5b834b..2d1f3c0f3c8f 100644
> > --- a/Documentation/driver-api/vfio-mediated-device.rst
> > +++ b/Documentation/driver-api/vfio-mediated-device.rst
> > @@ -202,6 +202,7 @@ Directories and files under the sysfs for Each Physical Device
> > | | |--- available_instances
> > | | |--- device_api
> > | | |--- description
> > + | | |--- migration_version
> > | | |--- [devices]
> > | |--- [<type-id>]
> > | | |--- create
> > @@ -209,6 +210,7 @@ Directories and files under the sysfs for Each Physical Device
> > | | |--- available_instances
> > | | |--- device_api
> > | | |--- description
> > + | | |--- migration_version
> > | | |--- [devices]
> > | |--- [<type-id>]
> > | |--- create
> > @@ -216,6 +218,7 @@ Directories and files under the sysfs for Each Physical Device
> > | |--- available_instances
> > | |--- device_api
> > | |--- description
> > + | |--- migration_version
> > | |--- [devices]
> >
> > * [mdev_supported_types]
> > @@ -246,6 +249,116 @@ 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.
>
> I've got only a few suggestions to improve to wording in the documentation
> (feel free to disagree):
>
hi Erik,
Thanks for your good suggestions. They are better to understand than
the original ones:)
I'll update the doc according to them except for below minor one --
may I just put it like this:
* migration_version (rw, optional)
Thanks
Yan
> >
> > +* migration_version
> > +
> > + This attribute is rw, and is optional.
>
> IMO better wording: "This is an optional, RW attribute."
>
> > + It is used to check migration compatibility between two mdev devices of the
> > + same mdev type. Absence of this attribute means the device of type <type-id>
> > + does not support migration.
> > + This attribute provides a way to check migration compatibility between two
> > + mdev devices from userspace even before device creation. The intended usage is
>
> ^This sentence essentially duplicates the information from the first sentence,
> can we condense it into something like:
>
> "It is used to check the migration compatibility between two mdev devices of the
> same mdev type. Typically, the target device has not been created yet at the
> time of userspace using this attribute to check the migration compatibility."
>
> > + for userspace to read the migration_version attribute from one mdev device and
> > + then writing that value to the migration_version attribute of the other mdev
> > + device. The second mdev device indicates compatibility via the return code of
> > + the write operation. This makes compatibility between mdev devices completely
> > + vendor-defined and opaque to userspace. Userspace should do nothing more
> > + than verify the mdev types match and then use the migration_version attribute
> > + to confirm source to target compatibility.
>
> I'd rephrase the ^last sentence differently:
> "Therefore, userspace is only expected to verify that the mdev types of the
> respective devices match and then use the migration_version attribute to
> confirm migration compatibility between the source and target mdev devices."
>
> > +
> > + Reading/Writing Attribute Data:
> > + read(2) will fail if device of type <type-id> does not support migration and
> > + otherwise succeed and return migration_version string of the device of
>
> "returns a migration_version string of the device on success, fails with an
> errno if the device doesn't support migration"
>
> > + type <type-id>.
> > +
> > + This migration_version string is vendor defined and opaque to the
> > + userspace. Vendor is free to include whatever they feel is relevant.
> > + e.g. <pciid of parent device>-<software version>.
> > +
> > + Restrictions on this migration_version string:
> > + 1. It should only contain ascii characters
> > + 2. MAX Length is PATH_MAX (4096)
> > +
> > + write(2) expects migration_version string of source mdev device, and will
> > + succeed if it is determined to be compatible and otherwise fail with
> > + vendor specific errno.
>
> "expects a migration_version string of the source mdev device, succeeds if the
> two mdev devices are migration compatible, otherwise fails with and errno"
>
> > +
> > + Errno:
> > + -An errno on read(2) indicates the device of type <type-id> does not support
> > + migration;
> > + -An errno on write(2) indicates the devices are incompatible or the target
> > + doesn't support migration.
> > + Vendor driver is free to define specific errno and is suggested to
> > + print detailed error in syslog for diagnose purpose.
> > +
> > + Userspace should treat ANY of below conditions as two mdev devices not
>
> Userspace should treat any of the below conditions as an indication of migration
> incompatibility between two mdev devices.
>
> > + compatible:
> > + (0) The mdev devices are not of the same type
> > + (1) any one of the two mdev devices does not have a migration_version
> > + attribute
>
> any of the two mdev devices is missing the migration_version attribute
>
> > + (2) error when reading from migration_version attribute of one mdev device
>
> when reading the source mdev's migration_version attribute
>
> > + (3) error when writing migration_version string of one mdev device to
> > + migration_version attribute of the other mdev device
>
> when writing the source mdev migration_version string to the target mdev
> device's migration_version attribute
>
> > +
> > + Userspace should regard two mdev devices compatible when ALL of below
> > + conditions are met:
>
> Userspace can consider the two mdev devices to be compatible when all of the
> below conditions are met:
>
> > + (0) The mdev devices are of the same type
> > + (1) success when reading from migration_version attribute of one mdev device.
>
> reading the migration_version attribute of the source succeeds
>
> > + (2) success when writing migration_version string of one mdev device to
> > + migration_version attribute of the other mdev device.
>
> writing the migration_version string to the target mdev's migration_version
> attribute succeeds
>
> > +
> > + Example Usage:
> > + (1) Compare mdev types:
>
> Comparing two 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 migration_version:
> > +
> > + The migration_version information can either be read from the mdev_type link
> > + on an instantiated device:
>
> s/information/string
>
> > +
> > + # cat /sys/bus/mdev/devices/$UUID1/mdev_type/migration_version
> > +
> > + Or it can be read from the mdev type definition, for example:
> > +
> > + # cat /sys/class/mdev_bus/*/mdev_supported_types/$MDEV_TYPE/migration_version
> > +
> > + If reading the source migration_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 migration_versions.
> > + Matching the specific mdev type to a parent may become important in such
> > + configurations.
> > +
> > + (3) Test source migration_version at target:
> > +
> > + Given a migration_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/migration_version
> > +
> > + If this write fails, the source and target migration versions are not
> > + compatible or the target does not support migration.
> > +
> > + Compatibility can also be tested prior to target device creation using the
>
> prior to creation of the target device
>
> > + mdev type definition for a parent device with a previously found matching mdev
> > + type, for example:
>
> using the migration_version attribute present inside a specific mdev type
> directory for a given physical parent device.
>
> > +
> > + # echo $VERSION > \
> > + /sys/class/mdev_bus/$PARENT/mdev_supported_types/$MDEV_TYPE/migration_version
> > +
> > + Again, an error writing the migration_version indicates that an instance of
> > + this mdev type would not support a migration from the provided migration
> > + version.
>
> would not support migration from the source.
>
> --
> Erik Skultety
>
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