RE: [virtio-dev] Re: [PATCH 0/5] virtio: introduce SUSPEND bit and vq state

Parav Pandit
2023-09-21T04:11:55Z

> From: Jason Wang <jasowang@redhat.com>
> Sent: Thursday, September 21, 2023 9:32 AM
>
> On Thu, Sep 21, 2023 at 11:51 AM Parav Pandit <parav@nvidia.com> wrote:
> >
> >
> >
> > > From: Jason Wang <jasowang@redhat.com>
> > > Sent: Thursday, September 21, 2023 8:45 AM The main issue I see so
> > > far is that you want to couple migration with admin commands but I
> > > don't see much advantages to doing this.
> > >
> > The way I read above comment is, to draw a parallel line: descriptor posting in
> virtio spec is tied to virtqueues. What is the advantage of it?
>
> Are you saying virtio can't live without admin commands? Again, let's not shift
> concepts.
>
No, I did not say that.
I just don’t see how functionalities proposed in [1] can be done without admin commands by the _device_ for member device passthrough requirement.

You made point as "don’t see much advantage with migration done using admin commands".
What is the advantage of descriptor posting using virtqueue. It is the way of virtio spec...

> > Well, it is one way to achieve it.
> > There may be different way to do all bulk data transfer without admin
> commands.
>
> Why is virtqueue the only way to do bulk data transferring? Can't DMA be
> initiated by other-way?
>

Sure, what is the disadvantage of existing mechanism of virtqueue that can do following.
1. Ability to do DMA
2. agnostic of the DMA who do not want to do DMA
3. Ability to multiple command executions in parallel
4. Non blocking interface for driver that does not require any kind of polling

Why to invent new DMA scheme which at does all the 4 tasks?
First please list down disadvantages of admin queue + show all 4 things are achieved using new DMA interface.
That will help to understand why new dma interface is needed.

> Thanks
>
> > What it the advantage of it, please list down them in [1] for the command
> where you can find alternative.
> >
> > [1]
> > https://lore.kernel.org/virtio-comment/20230909142911.524407-7-parav@n
> > vidia.com/T/#md9fcfa1ba997463de8c7fb8c6d1786b224b0bead