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Re: [ovirt-devel] Re: device compatibility interface for live migration with assigned devices

On 2020/8/21 下午10:52, Cornelia Huck wrote:
On Fri, 21 Aug 2020 11:14:41 +0800
Jason Wang <jasowang redhat com> wrote:

On 2020/8/20 下午8:27, Cornelia Huck wrote:
On Wed, 19 Aug 2020 17:28:38 +0800
Jason Wang <jasowang redhat com> wrote:
On 2020/8/19 下午4:13, Yan Zhao wrote:
On Wed, Aug 19, 2020 at 03:39:50PM +0800, Jason Wang wrote:
On 2020/8/19 下午2:59, Yan Zhao wrote:
On Wed, Aug 19, 2020 at 02:57:34PM +0800, Jason Wang wrote:
On 2020/8/19 上午11:30, Yan Zhao wrote:
hi All,
could we decide that sysfs is the interface that every VFIO vendor driver
needs to provide in order to support vfio live migration, otherwise the
userspace management tool would not list the device into the compatible

if that's true, let's move to the standardizing of the sysfs interface.
(1) content
common part: (must)
        - software_version: (in major.minor.bugfix scheme)
This can not work for devices whose features can be negotiated/advertised
independently. (E.g virtio devices)
I thought the 'software_version' was supposed to describe kind of a
'protocol version' for the data we transmit? I.e., you add a new field,
you bump the version number.

Ok, but since we mandate backward compatibility of uABI, is this really
worth to have a version for sysfs? (Searching on sysfs shows no examples
like this)
I was not thinking about the sysfs interface, but rather about the data
that is sent over while migrating. E.g. we find out that sending some
auxiliary data is a good idea and bump to version 1.1.0; version 1.0.0
cannot deal with the extra data, but version 1.1.0 can deal with the
older data stream.


Well, I think what data to transmit during migration is the duty of qemu not kernel. And I suspect the idea of reading opaque data (with version) from kernel and transmit them to dest is the best approach.

        - device_api: vfio-pci or vfio-ccw ...
        - type: mdev type for mdev device or
                a signature for physical device which is a counterpart for
	   mdev type.

device api specific part: (must)
       - pci id: pci id of mdev parent device or pci id of physical pci
         device (device_api is vfio-pci)API here.
So this assumes a PCI device which is probably not true.
for device_api of vfio-pci, why it's not true?

for vfio-ccw, it's subchannel_type.
Ok but having two different attributes for the same file is not good idea.
How mgmt know there will be a 3rd type?
that's why some attributes need to be common. e.g.
device_api: it's common because mgmt need to know it's a pci device or a
               ccw device. and the api type is already defined vfio.h.
	    (The field is agreed by and actually suggested by Alex in previous mail)
type: mdev_type for mdev. if mgmt does not understand it, it would not
         be able to create one compatible mdev device.
software_version: mgmt can compare the major and minor if it understands
         this fields.
I think it would be helpful if you can describe how mgmt is expected to
work step by step with the proposed sysfs API. This can help people to
My proposal would be:
- check that device_api matches
- check possible device_api specific attributes
- check that type matches [I don't think the combination of mdev types
    and another attribute to determine compatibility is a good idea;

Any reason for this? Actually if we only use mdev type to detect the
compatibility, it would be much more easier. Otherwise, we are actually
re-inventing mdev types.

E.g can we have the same mdev types with different device_api and other
In the end, the mdev type is represented as a string; but I'm not sure
we can expect that two types with the same name, but a different
device_api are related in any way.

If we e.g. compare vfio-pci and vfio-ccw, they are fundamentally

I was mostly concerned about the aggregation proposal, where type A +
aggregation value b might be compatible with type B + aggregation value

Yes, that looks pretty complicated.

    actually, the current proposal confuses me every time I look at it]
- check that software_version is compatible, assuming semantic
- check possible type-specific attributes

I'm not sure if this is too complicated. And I suspect there will be
vendor specific attributes:

- for compatibility check: I think we should either modeling everything
via mdev type or making it totally vendor specific. Having something in
the middle will bring a lot of burden
FWIW, I'm for a strict match on mdev type, and flexibility in per-type

I'm not sure whether the above flexibility can work better than encoding them to mdev type. If we really want ultra flexibility, we need making the compatibility check totally vendor specific.

- for provisioning: it's still not clear. As shown in this proposal, for
NVME we may need to set remote_url, but unless there will be a subclass
(NVME) in the mdev (which I guess not), we can't prevent vendor from
using another attribute name, in this case, tricks like attributes
iteration in some sub directory won't work. So even if we had some
common API for compatibility check, the provisioning API is still vendor
specific ...
Yes, I'm not sure how to deal with the "same thing for different
vendors" problem. We can try to make sure that in-kernel drivers play
nicely, but not much more.

Then it's actually a subclass of mdev I guess in the future.


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