Re: device compatibility interface for live migration with assigned devices

[Jason Wang <jasowang@redhat.com>]

On 2020/7/18 上午12:12, Alex Williamson wrote:
> On Thu, 16 Jul 2020 16:32:30 +0800
> Yan Zhao <yan.y.zhao@intel.com> wrote:
>
>> On Thu, Jul 16, 2020 at 12:16:26PM +0800, Jason Wang wrote:
>>> On 2020/7/14 上午7:29, Yan Zhao wrote:
>>>> hi folks,
>>>> we are defining a device migration compatibility interface that helps upper
>>>> layer stack like openstack/ovirt/libvirt to check if two devices are
>>>> live migration compatible.
>>>> The "devices" here could be MDEVs, physical devices, or hybrid of the two.
>>>> e.g. we could use it to check whether
>>>> - a src MDEV can migrate to a target MDEV,
>>>> - a src VF in SRIOV can migrate to a target VF in SRIOV,
>>>> - a src MDEV can migration to a target VF in SRIOV.
>>>>     (e.g. SIOV/SRIOV backward compatibility case)
>>>>
>>>> The upper layer stack could use this interface as the last step to check
>>>> if one device is able to migrate to another device before triggering a real
>>>> live migration procedure.
>>>> we are not sure if this interface is of value or help to you. please don't
>>>> hesitate to drop your valuable comments.
>>>>
>>>>
>>>> (1) interface definition
>>>> The interface is defined in below way:
>>>>
>>>>                __    userspace
>>>>                 /\              \
>>>>                /                 \write
>>>>               / read              \
>>>>      ________/__________       ___\|/_____________
>>>>     | migration_version |     | migration_version |-->check migration
>>>>     ---------------------     ---------------------   compatibility
>>>>        device A                    device B
>>>>
>>>>
>>>> a device attribute named migration_version is defined under each device's
>>>> sysfs node. e.g. (/sys/bus/pci/devices/0000\:00\:02.0/$mdev_UUID/migration_version).
>>>
>>> Are you aware of the devlink based device management interface that is
>>> proposed upstream? I think it has many advantages over sysfs, do you
>>> consider to switch to that?
>
> Advantages, such as?


My understanding for devlink(netlink) over sysfs (some are mentioned at 
the time of vDPA sysfs mgmt API discussion) are:

- existing users (NIC, crypto, SCSI, ib), mature and stable
- much better error reporting (ext_ack other than string or errno)
- namespace aware
- do not couple with kobject

Thanks