Re: [PATCH vfio 10/11] vfio/virtio: Expose admin commands over virtio device

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On Wed, Sep 27, 2023 at 10:18:17AM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 26, 2023 at 07:41:44AM -0400, Michael S. Tsirkin wrote:
> > > By the way, this follows what was done already between vfio/mlx5 to
> > > mlx5_core modules where mlx5_core exposes generic APIs to execute a command
> > > and to get the a PF from a given mlx5 VF.
> > 
> > This is up to mlx5 maintainers. In particular they only need to worry
> > that their patches work with specific hardware which they likely have.
> > virtio has to work with multiple vendors - hardware and software -
> > and exposing a low level API that I can't test on my laptop
> > is not at all my ideal.
> mlx5 has a reasonable API from the lower level that allows the vfio
> driver to safely issue commands. The API provides all the safety and
> locking you have been questioning here.
> Then the vfio driver can form the commands directly and in the way it
> needs. This avoids spewing code into the core modules that is only
> used by vfio - which has been a key design consideration for our
> driver layering.
> I suggest following the same design here as it has been well proven.
> Provide a solid API to operate the admin queue and let VFIO use
> it. One of the main purposes of the admin queue is to deliver commands
> on behalf of the VF driver, so this is a logical and reasonable place
> to put an API.

Not the way virtio is designed now. I guess mlx5 is designed in
a way that makes it safe.

> > > This way, we can enable further commands to be added/extended
> > > easily/cleanly.
> > 
> > Something for vfio maintainer to consider in case it was
> > assumed that it's just this one weird thing
> > but otherwise it's all generic vfio. It's not going to stop there,
> > will it? The duplication of functionality with vdpa will continue :(
> VFIO live migration is expected to come as well once OASIS completes
> its work.

Exactly. Is there doubt vdpa will want to support live migration?
Put this code in a library please.

> Parav, are there other things?
> Jason

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