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Re: VFIO Migration
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From: |
Jason Wang |
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Subject: |
Re: VFIO Migration |
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Date: |
Wed, 4 Nov 2020 11:32:34 +0800 |
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Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 |
On 2020/11/3 下午8:15, Stefan Hajnoczi wrote:
On Tue, Nov 03, 2020 at 04:46:53PM +0800, Jason Wang wrote:
On 2020/11/2 下午7:11, Stefan Hajnoczi wrote:
There is discussion about VFIO migration in the "Re: Out-of-Process
Device Emulation session at KVM Forum 2020" thread. The current status
is that Kirti proposed a VFIO device region type for saving and loading
device state. There is currently no guidance on migrating between
different device versions or device implementations from different
vendors. This is known to be non-trivial and raised discussion about
whether it should really be handled by VFIO or centralized in QEMU.
Below is a document that describes how to ensure migration compatibility
in VFIO. It does not require changes to the VFIO migration interface. It
can be used for both VFIO/mdev kernel devices and vfio-user devices.
The idea is that the device state blob is opaque to the VMM but the same
level of migration compatibility that exists today is still available.
So if we can't mandate this or there's no way to validate this. Vendor is
still free to implement their own protocol which could lead a lot of
maintaining burden.
Yes, the device state representation is their responsibility. We can't
do that for them since they define the hardware interface and internal
state.
As Michael and Paolo have mentioned in the other thread, we can provide
guidelines and standardize common aspects.
Migration can fail if loading the device state is not possible. It should fail
early with a clear error message. It must not appear to complete but leave the
device inoperable due to a migration problem.
For VFIO-user, how management know that a VM can be migrated from src to
dst? For kernel, we have sysfs.
vfio-user devices will normally be instantiated in one of two ways:
1. Launching a device backend and passing command-line parameters:
$ my-nic --socket-path /tmp/my-nic-vfio-user.sock \
--model https://vendor-a.com/my-nic \
--rss on
Here "model" is the device model URL. The program could support
multiple device models.
The "rss" device configuration parameter enables Receive Side Scaling
(RSS) as an example of a configuration parameter.
2. Creating a device using an RPC interface:
(qemu) device-add my-nic,rss=on
If the device instantiation succeeds then it is safe to live migrate.
The device is exposing the desired hardware interface and expecting the
right device state representation.
Does this mean there will still be a "my-nic" stub in qemu? (I thought
it should be a generic one like device-add "vfio-user-pci")
The rest of this document describes how these requirements can be met.
Device Models
-------------
Devices have a *hardware interface* consisting of hardware registers,
interrupts, and so on.
The hardware interface together with the device state representation is called
a *device model*. Device models can be assigned URIs such as
https://qemu.org/devices/e1000e to uniquely identify them.
It looks worse than "pci://vendor_id.device_id.subvendor_id.subdevice_id".
"e1000e" means a lot of different 8275X implementations that have subtle but
easy to be ignored differences.
If you wish to reflect those differences in the device model URI then
you can use:
https://qemu.org/devices/pci/<vendor-id>/<device-id>/<subvendor-id>/<subdevice-id>
Another option is to use device configuration parameters to express
differences.
The important thing is that this device model URI has one owner. No one
else will use qemu.org. There can be many different e1000e device model
URIs, if necessary (with slightly different hardware interfaces and/or
device state representations). This avoids collisions.
And is it possible to have a list of URIs here?
A device implementation (mdev driver, vfio-user device backend, etc) may
support multiple device model URIs.
A device instance has an immutable device model URI and list of
configuration parameters. In other words, once the device is created its
ABI is fixed for the lifetime of the device. A new device instance can
be configured by powering off the machine, hotplug, etc.
Multiple implementations of a device model may exist. They are they are
interchangeable if they follow the same hardware interface and device
state representation.
Multiple implementations of the same hardware interface may exist with
different device state representations, in which case the device models are not
interchangeable and must be assigned different URIs.
Migration is only possible when the same device model is supported by the
*source* and the *destination* devices.
Device Configuration
--------------------
Device models may have parameters that affect the hardware interface or device
state representation. For example, a network card may have a configurable
address filtering table size parameter called ``rx-filter-size``. A
device state saved with ``rx-filter-size=32`` cannot be safely loaded
into a device with ``rx-filter-size=0``, because changing the size from
32 to 0 may disrupt device operation.
Do we allow the migration from "rx-filter-size=16" to "rx-filter-size=32" (I
guess not?) And should we extend the concept to "device capability" instead
of just state representation. E.g src has CAP_X=on,CAP_Y=off but dst has
CAP_X=on,CAP_Y=on, so we disallow the migration from src to dst.
A device instance's configuration parameters are immutable.
rx-filter-size=16 cannot be migrated to rx-filter-size=32.
But then it looks to me we can't migrate back, or do you mean it is
required to have the ability to change the max rx-filter-size.
Yes, configuration parameters can describe capabilities. I think of
capabilities as something that affects the guest-visible hardware
interface (e.g. the RSS feature bit is enabled) that is mentioned in the
text, but it would be clearer to mention them explicitly.
A list of configuration parameters is called the *device configuration*.
Migration is expected to succeed when the same device model and configuration
that was used for saving the device state is used again to load it.
Note that not all parameters used to instantiate a device need to be part of
the device configuration. For example, assigning a network card to a specific
physical port is not part of the device configuration since it is not part of
the device's hardware interface or the device state representation.
Yes, but the task needs to be done by management somehow. So do you expect a
vendor specific provisioning API here?
There seems to be no consensus on this yet. It's the question of how to
manage the lifecycle of VFIO, mdev, vhost-user, and vfio-user devices.
There are attempts to standardize in some of these areas.
For mdev drivers we can standardize the sysfs interface so management
tools can query source devices and instantiate destination devices
without device-specific code.
Even for mdev, it should be have some class defined for sysfs which
could be a standard way to configure NVME or virtio device.
For vhost-user devices there is the backend program conventions
specification, which aims to standardize common parameters. This makes
integrating support for new device implementations easier (there is less
device implementation-specific code).
For vfio-user devices something based on the vhost-user backend program
conventions spec could work well.
The main issue could be that avoiding vendor-specific provisioning code
in management software either requires you to restrict yourself to a few
standard device types or to pass through configuration data.
A libvirt opinion would be interesting.
The device
state can be loaded and run on a different physical port without affecting the
operation of the device. Therefore the physical port is not part of the device
configuration.
However, secondary aspects related to the physical port may affect the device's
hardware interface and need to be reflected in the device configuration. The
link speed may depend on the physical port and be reported through the device's
hardware interface. In that case a ``link-speed`` configuration parameter is
required to prevent unexpected changes to the link speed after migration.
Note that the device configuration is a conservative bound on device
states that can be migrated successfully since not all configuration
parameters may be strictly required to match on the source and
destination devices. For example, if the device's hardware interface has
not yet been initialized then changes to the link speed may not be
noticed. However, accurately representing runtime constraints is complex
and risks introducing migration bugs, so no attempt is made to support
them to achieve more relaxed bounds on successful migrations.
Device Versions
---------------
As a device evolves, the number of configuration parameters required may become
inconvenient for users to express in full. A device configuration can be
aliased by a *device version*, which is a shorthand for the full device
configuration. This makes it easy to apply a standard device configuration
without listing every configuration parameter explicitly.
I'm not sure how to apply the device versions consider the device state is
opaque or the device needs to export another API to do this?
Versions are just aliases for a list of configuration parameters. For
example, version=2 expands to rx-filter-size=32. The purpose of versions
is to provide a human-readable shorthand notation.
Versions are not involved in migration compatibility checking, instead
the device model URI and expanded configuration parameters are compared.
The version has no direct effect on the device state representation. It
has an indirect effect due to the configuration parameters that it
expands to. For example, the rx-filter-size=32 configuration parameter
may change the device state representation to include the 32 addresses
that the device is filtering on.
No "version check" is necessary when loading the device state
representation because the device was already instantiated with the
exact configuration parameters that determine the device state
representation.
For example, if address filtering support was added to a network card then
device versions and the corresponding configurations may look like this:
* ``version=1`` - Behaves as if ``rx-filter-size=0``
* ``version=2`` - ``rx-filter-size=32``
Device States
-------------
The details of the device state representation are not covered in this document
but the general requirements are discussed here.
The device state consists of data accessible through the device's hardware
interface and internal state that is needed to restore device operation.
State in the hardware interface includes the values of hardware registers.
An example of internal state is an index value needed to avoid processing
queued requests more than once.
Changes can be made to the device state representation as follows. Each change
to device state must have a corresponding device configuration parameter that
allows the change to toggled:
* When the parameter is disabled the hardware interface and device state
representation are unchanged. This allows old device states to be loaded.
* When the parameter is enabled the change comes into effect.
* The parameter's default value disables the change. Therefore old versions do
not have to explicitly specify the parameter.
The following example illustrates migration from an old device
implementation to a new one. A version=1 network card is migrated to a
new device implementation that is also capable of version=2 and adds the
rx-filter-size=32 parameter. The new device is instantiated with
version=1, which disables rx-filter-size and is capable of loading the
version=1 device state. The migration completes successfully but note
the device is still operating at version=1 level in the new device.
The following example illustrates migration from a new device
implementation back to an older one. The new device implementation
supports version=1 and version=2. The old device implementation supports
version=1 only. Therefore the device can only be migrated when
instantiated with version=1 or the equivalent full configuration
parameters.
In qemu we have subsection to facilitate the case when some fields were
forgot to migrate. Do we need something similar here?
This is an important question and I'm not sure.
The problem with subsection semantics is that they break rollback. Once
the old device state has been loaded by the new device implementation,
saving the device state produces the new device state representation.
The old device implementation can no longer load it :(.
Only when subsection is needed.
Manual
intervention is necessary to tell the new device implementation to save
in the old representation.
If we don't support subsection, could we end up with a deadlock like we
do migration since want upgrade the kernel, but if we don't upgrade the
kernel, we can't do live migration.
In the migration model described in this document it works the other
way around: back and forth migration is always safe. If you wish to
change the device you need to create a new instance (after poweroff or
through hotplug).
One way of achieving something similar is to provide additional
information about safe transitions between configuration parameter
lists. It is not safe to change arbitrary device configuration
parameters, but certain parameters can be safely changed.
I'm not sure if the complexity is worth it though. The downside to the
current approach is that devices must eventually be reconfigured to
upgrade to new versions, even if there is no guest-visible hardware
interface change.
Stefan
Thanks
- Re: VFIO Migration, (continued)
Re: VFIO Migration, Alex Williamson, 2020/11/02
Re: VFIO Migration, Jason Wang, 2020/11/03
Re: VFIO Migration, Daniel P . Berrangé, 2020/11/03
Re: VFIO Migration, Dr. David Alan Gilbert, 2020/11/03