Re: [virtio-comment] Re: [PATCH v1 3/8] device-context: Define the device context fields for device migration

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 23, 2023 at 11:53:50AM +0800, Jason Wang wrote:
> On Fri, Oct 20, 2023 at 5:41âPM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Fri, Oct 20, 2023 at 05:31:01PM +0800, Zhu, Lingshan wrote:
> > >
> > >
> > > On 10/19/2023 6:33 PM, Parav Pandit wrote:
> > > > > From: Zhu, Lingshan <lingshan.zhu@intel.com>
> > > > > Sent: Thursday, October 19, 2023 2:48 PM
> > > > >
> > > > > On 10/19/2023 5:14 PM, Michael S. Tsirkin wrote:
> > > > > > On Thu, Oct 19, 2023 at 09:13:16AM +0000, Parav Pandit wrote:
> > > > > > > > Oh, really? Quite interesting, do you want to move all config space
> > > > > > > > fields in VF to admin vq? Have a plan?
> > > > > > > Not in my plan for spec 1.4 time frame.
> > > > > > > I do not want to divert the discussion, would like to focus on device
> > > > > migration phases.
> > > > > > > Lets please discuss in some other dedicated thread.
> > > > > > Possibly, if there's a way to send admin commands to vf itself then
> > > > > > Lingshan will be happy?
> > > > > still need to prove why admin commands are better than registers.
> > > > Virtio spec development is not proof based approach. Please stop asking for it.
> > > >
> > > > I tried my best to have technical answer in [1].
> > > > I explained that registers simply do not work for passthrough mode
> > > > (if this is what you are asking when you are asking prove its better).
> > > > They can work for non_passthrough mediated mode.
> > > >
> > > > A member device may do admin commands using registers. Michael and I are discussing presently in the same thread.
> > > >
> > > > Since there are multiple things to be done for device migration, dedicated register set for each functionality do not scale well, hard to maintain and extend.
> > > > A register holding a command content make sense.
> > > >
> > > > Now, with that, if this can be useful only for non_passthrough, I made humble request to transport them using AQ, this way, you get all benefits of AQ.
> > > > And trying to understand, why AQ cannot possible or inferior?
> > > >
> > > > If you have commands like suspend/resume device, register or queue transport simply donât work, because it's wrong to bifurcate the device with such weird API.
> > > > If you want to biferacate for mediation software, it probably makes sense to operate at each VQ level, config space level. Such are very different commands than passthrough.
> > > > I think vdpa has demonstrated that very well on how to do specific work for specific device type. So some of those work can be done using AQ.
> > > >
> > > > [1] 870ace02-f99c-4582-932f-bd103362dae9@intel.com/T/#m37743aa924536d0256d6b3b8e83a11c750f28794">https://lore.kernel.org/virtio-comment/870ace02-f99c-4582-932f-bd103362dae9@intel.com/T/#m37743aa924536d0256d6b3b8e83a11c750f28794
> > > We have been through your statement for many times.
> > > This is not about how many times you repeated, if you think this is true,
> > > you need to prove that with solid evidence.
> > >
> > >
> > > For pass-through, I still recommend you to take a reference of current
> > > virito-pci implementation, it works for pass-through, right?
> >
> > Current migration implementation in e.g. QEMU? It does but it
> > traps data path accesses. That, I think we can agree,
> > should not be the only option to migrate.
> >
> > > For scale, I already told you for many times that they are per-device
> > > facilities. How can a per-device facility not scale?
> > > vDPA works fine on config space.
> > >
> > > So, if you still insist admin vq is better than config space like in other
> > > thread you have concluded, you may imply that config space interfaces should
> > > be re-factored to admin vq.
> >
> > There are good arguments that yes, virtio needs a transport for config space
> > that is DMA based as opposed to memory mapped based.  This is one of the
> > things all vendors seem to prefer in IDPF so virtio should have the option.
> 
> Then it is the transport vq or transport over adminq proposal?
> 
> Thanks

That's one way to do it, yes.

-- 
MST