Re: [PATCH v4 2/3] ethdev: add standby state for live migration

[Thomas Monjalon <thomas@monjalon.net>]

01/02/2023 08:32, Andrew Rybchenko:
> On 2/1/23 01:55, Thomas Monjalon wrote:
> > 31/01/2023 19:14, Jerin Jacob:
> >> On Wed, Jan 18, 2023 at 9:15 PM Rongwei Liu <rongweil@nvidia.com> wrote:
> >>>
> >>> When a DPDK application must be upgraded,
> >>> the traffic downtime should be shortened as much as possible.
> >>> During the migration time, the old application may stay alive
> >>> while the new application is starting and being configured.
> >>>
> >>> In order to optimize the switch to the new application,
> >>> the old application may need to be aware of the presence
> >>> of the new application being prepared.
> >>> This is achieved with a new API allowing the user to change the
> >>> new application state to standby and active later.
> >>>
> >>> The added function is trying to apply the new state to all probed
> >>> ethdev ports. To make this API simple and easy to use,
> >>> the same flags have to be accepted by all devices.
> >>>
> >>> This is the scenario of operations in the old and new applications:
> >>> .       device: already configured by the old application
> >>> .       new:    start as active
> >>> .       new:    probe the same device
> >>
> >> How to probe same device if is already bind to another application?
> >> vfio-pci wont allow this.
> > 
> > I missed that part.
> > There is no way to share a VFIO device between 2 applications?
> 
> As I understand multi-process shares an VFIO device between
> many application. As far as I remember it is just required to
> pass corresponding file descriptor to another application.
> 
> Anyway I fully agree that the patch requires more documentation
> in doc/guides/ with the description of live migration theory of
> operations.

Yes that part is missing.
Rongwei, you should try a fake migration with e1000 or virtio
(both available in QEMU), so you can play with VFIO device probing.

> >>> .       new:    set as standby
> >>> .       new:    configure the device
> >>> .       device: has configurations from old and new applications
> >>> .       old:    clear its device configuration
> >>> .       device: has only 1 configuration from new application
> >>> .       new:    set as active
> >>> .       device: downtime for connecting all to the new application
> >>> .       old:    shutdown