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Subject: Re: [virtio-comment] Live Migration of Virtio Virtual Function



On 8/17/2021 12:44 PM, Jason Wang wrote:
On Tue, Aug 17, 2021 at 5:11 PM Max Gurtovoy <mgurtovoy@nvidia.com> wrote:

On 8/17/2021 11:51 AM, Jason Wang wrote:
å 2021/8/12 äå8:08, Max Gurtovoy åé:
Hi all,

Live migration is one of the most important features of
virtualization and virtio devices are oftenly found in virtual
environments.

The migration process is managed by a migration SW that is running on
the hypervisor and the VM is not aware of the process at all.

Unlike the vDPA case, a real pci Virtual Function state resides in
the HW.

vDPA doesn't prevent you from having HW states. Actually from the view
of the VMM(Qemu), it doesn't care whether or not a state is stored in
the software or hardware. A well designed VMM should be able to hide
the virtio device implementation from the migration layer, that is how
Qemu is wrote who doesn't care about whether or not it's a software
virtio/vDPA device or not.


In our vision, in order to fulfil the Live migration requirements for
virtual functions, each physical function device must implement
migration operations. Using these operations, it will be able to
master the migration process for the virtual function devices. Each
capable physical function device has a supervisor permissions to
change the virtual function operational states, save/restore its
internal state and start/stop dirty pages tracking.

For "supervisor permissions", is this from the software point of view?
Maybe it's better to give an example for this.
A permission to a PF device for quiesce and freeze a VF device for example.
Note that for safety, VMM (e.g Qemu) is usually running without any privileges.

You're mixing layers here.

QEMU is not involved here. It's only sending IOCTLs to migration driver. The migration driver will control the migration process of the VF using the PF communication channel.




An example of this approach can be seen in the way NVIDIA performs
live migration of a ConnectX NIC function:

https://github.com/jgunthorpe/linux/commits/mlx5_vfio_pci
<https://github.com/jgunthorpe/linux/commits/mlx5_vfio_pci>

NVIDIAs SNAP technology enables hardware-accelerated software defined
PCIe devices. virtio-blk/virtio-net/virtio-fs SNAP used for storage
and networking solutions. The host OS/hypervisor uses its standard
drivers that are implemented according to a well-known VIRTIO
specifications.

In order to implement Live Migration for these virtual function
devices, that use a standard drivers as mentioned, the specification
should define how HW vendor should build their devices and for SW
developers to adjust the drivers.

This will enable specification compliant vendor agnostic solution.

This is exactly how we built the migration driver for ConnectX
(internal HW design doc) and I guess that this is the way other
vendors work.

For that, I would like to know if the approach of âPF that controls
the VF live migration processâ is acceptable by the VIRTIO technical
group ?

I'm not sure but I think it's better to start from the general
facility for all transports, then develop features for a specific
transport.
a general facility for all transports can be a generic admin queue ?
It could be a virtqueue or a transport specific method (pcie capability).

No. You said a general facility for all transports.

Transport specific is not general.


E.g we can define what needs to be migrated for the virtio-blk first
(the device state). Then we can define the interface to get and set
those states via admin virtqueue. Such decoupling may ease the future
development of the transport specific migration interface.

I asked a simple question here.

Lets stick to this.

I'm not referring to internal state definitions.

Can you please not change the subject of my initial intent in the email ?

Thanks.



Thanks


Thanks


Cheers,

-Max.

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