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 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 don’t 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] 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.


-- 
MST


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