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

On 10/21/2023 11:34 PM, Michael S. Tsirkin wrote:
On Fri, Oct 20, 2023 at 07:11:49PM +0800, Zhu, Lingshan wrote:

On 10/20/2023 5:41 PM, Michael S. Tsirkin 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.
OK, I am glad we agree that config space work for pass-through,
hope we don't need to discuss this anymore.
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.
Do you really want to refactor virtio-pci common config fields to PF's admin
E.g, do you really want to move queue_enable in virtio-pci common config
fields to PF's admin vq?
No, I think we need a transport with as small # of memory mapped registers as
possible that passes admin commands through the VF itself.
Then do you believe admin commands are better than registers in control path? This is an identical question to the above one, do you want to replace current virtio common cfg with admin commands? Do you want to use admin commands to process queue_enable
other than queeu_enable register?

config space, MMIO, registers work for years, what is wrong with them?

Config space is control path, DMA is data-path, let's better not mix them,
we never expect to use config space to transfer data.

So we need DMA to transfer data, for example I take advantages of device DMA
to logging dirty pages, This also applies to in-flight descriptors.
As long as you do, I personally see little benefit to retrieve parts of
state with memory mapped accesses.
registers only control, and I personally believe a single register is much better than processing admin commands, more light-weight, more reliable, working for years.

Config space interfaces are fundamental for virtio-pci.

And we are implementing virito live migration, not only for PCI.

So both me and Jason keep repeating: We are implementing basic facilities,
and the implementation is transport specific.
But the register based facilities you proposed are extremely limited and
seem to only work for migration. For example, it seems mostly useless for
debugging because retrieving state is rather complex and would
interfere with normal working of the device.
If you want to prove the register controlling interfaces are extremely limited than admin vq or admin cmds,
you are also proving config space registers are extremely limited than
admin vq.

So the question still here: do you want to replace current virtio-pci common cfg
with admin vq or admin cmds?

And debug what? If you want to introduce more functionalities, we should discuss
case by case.

If debugging vq state, it is as easy as read queue_size, I don't see the limitations
as queue_size work for years.

I still believe our goal is to do our best, with our capabilities, to build the most optimal virtio spec
as we can do. Not other goals.

Zhu Lingshan

We have proposed to build admin vq based on our register solution, this can
somehow even help tp resolve the nested issue.

But I see the proposed has been rejected.

I still believe the goal is to build a best spec, not "just can work" with

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