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Re: device compatibility interface for live migration with assigned devices




On 2020/8/14 下午1:16, Yan Zhao wrote:
On Thu, Aug 13, 2020 at 12:24:50PM +0800, Jason Wang wrote:
On 2020/8/10 下午3:46, Yan Zhao wrote:
driver is it handled by?
It looks that the devlink is for network device specific, and in
devlink.h, it says
include/uapi/linux/devlink.h - Network physical device Netlink
interface,

Actually not, I think there used to have some discussion last year and the
conclusion is to remove this comment.

It supports IB and probably vDPA in the future.

hmm... sorry, I didn't find the referred discussion. only below discussion
regarding to why to add devlink.

https://www.mail-archive.com/netdev vger kernel org/msg95801.html
	>This doesn't seem to be too much related to networking? Why can't something
	>like this be in sysfs?
	
	It is related to networking quite bit. There has been couple of
	iteration of this, including sysfs and configfs implementations. There
	has been a consensus reached that this should be done by netlink. I
	believe netlink is really the best for this purpose. Sysfs is not a good
	idea


See the discussion here:

https://patchwork.ozlabs.org/project/netdev/patch/20191115223355 1277139-1-jeffrey t kirsher intel com/



https://www.mail-archive.com/netdev vger kernel org/msg96102.html
	>there is already a way to change eth/ib via
	>echo 'eth' > /sys/bus/pci/drivers/mlx4_core/0000:02:00.0/mlx4_port1
	>
	>sounds like this is another way to achieve the same?
	
	It is. However the current way is driver-specific, not correct.
	For mlx5, we need the same, it cannot be done in this way. Do devlink is
	the correct way to go.

https://lwn.net/Articles/674867/
	There a is need for some userspace API that would allow to expose things
	that are not directly related to any device class like net_device of
	ib_device, but rather chip-wide/switch-ASIC-wide stuff.

	Use cases:
	1) get/set of port type (Ethernet/InfiniBand)
	2) monitoring of hardware messages to and from chip
	3) setting up port splitters - split port into multiple ones and squash again,
	   enables usage of splitter cable
	4) setting up shared buffers - shared among multiple ports within one chip



we actually can also retrieve the same information through sysfs, .e.g

|- [path to device]
   |--- migration
   |     |--- self
   |     |   |---device_api
   |	|   |---mdev_type
   |	|   |---software_version
   |	|   |---device_id
   |	|   |---aggregator
   |     |--- compatible
   |     |   |---device_api
   |	|   |---mdev_type
   |	|   |---software_version
   |	|   |---device_id
   |	|   |---aggregator


Yes but:

- You need one file per attribute (one syscall for one attribute)
- Attribute is coupled with kobject

All of above seems unnecessary.

Another point, as we discussed in another thread, it's really hard to make sure the above API work for all types of devices and frameworks. So having a vendor specific API looks much better.



   I feel like it's not very appropriate for a GPU driver to use
this interface. Is that right?

I think not though most of the users are switch or ethernet devices. It
doesn't prevent you from inventing new abstractions.
so need to patch devlink core and the userspace devlink tool?
e.g. devlink migration


It quite flexible, you can extend devlink, invent your own or let mgmt to establish devlink directly.



Note that devlink is based on netlink, netlink has been widely used by
various subsystems other than networking.
the advantage of netlink I see is that it can monitor device status and
notify upper layer that migration database needs to get updated.


I may miss something, but why this is needed?

From device point of view, the following capability should be sufficient to support live migration:

- set/get device state
- report dirty page tracking
- set/get capability


But not sure whether openstack would like to use this capability.
As Sean said, it's heavy for openstack. it's heavy for vendor driver
as well :)


Well, it depends several factors. Just counting LOCs, sysfs based attributes is not lightweight.

Thanks



And devlink monitor now listens the notification and dumps the state
changes. If we want to use it, need to let it forward the notification
and dumped info to openstack, right?

Thanks
Yan



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