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



On 10/11/2023 6:20 PM, Michael S. Tsirkin wrote:

On Mon, Oct 09, 2023 at 06:01:42PM +0800, Zhu, Lingshan wrote:


On 9/27/2023 11:40 PM, Michael S. Tsirkin wrote:

On Wed, Sep 27, 2023 at 04:20:01PM +0800, Zhu, Lingshan wrote:

On 9/26/2023 6:48 PM, Michael S. Tsirkin wrote:

On Tue, Sep 26, 2023 at 05:25:42PM +0800, Zhu, Lingshan wrote:

We don't want to repeat the discussions, it looks like endless circle with
no direction.

OK let me try to direct this discussion.
You guys were speaking past each other, no dialog is happening.
And as long as it goes on no progress will be made and you
will keep going in circles.

Parav here made an effort and attempted to summarize
use-cases addressed by your proposal but not his.
He couldn't resist adding "a yes but" in there oh well.
But now I hope you know he knows about your use-cases?

So please do the same. Do you see any advantages to Parav's
proposal as compared to yours? Try to list them and
if possible try not to accompany the list with "yes but"
(put it in a separate mail if you must ;) ).
If you won't be able to see any, let me know and I'll try to help.

Once each of you and Parav have finally heard the other and
the other also knows he's been heard, that's when we can
try to make progress by looking for something that addresses
all use-cases as opposed to endlessly repeating same arguments.

Sure Michael, I will not say "yes but" here.

  From Parav's proposal, he intends to migrate a member device by its owner
device through the admin vq,
thus necessary admin vq commands are introduced in his series.


I see his proposal can:
1) meet some customers requirements without nested and bare-metal
2) align with Nvidia production
3) easier to emulate by onboard SOC

Is that all you can see?

Hint: there's more.

please help provide more.

Just a small subset off the top of my head:
Error handling.

handle failed live migration? how?

and for other errors, we have mature error handling solutions
in virtio for years, like re-read, NEEDS_RESET.

If that is not good enough, then the corollary is:
admin vq is better than config space,
then the further corollary could be:
we should refactor virito-pci interfaces to admin vq commands,
like how we handle features

Is that true?

Extendable to other group types such as SIOV.

For SIOV, the admin vq is a transport, but for SR-IOV
the admin vq is a control channel, that is different,
and admin vq can be a side channel.

For example, for SIOV, we config and migrate MSIX through
admin vq. For SRIOV, they are in config space.

Batching of commands
less pci transactioons

so this can still be a QOS issue.
If batching, others to starve?

Support for keeping some data off-device

I don't get it, what is off-device?
The live migration facilities need to fetch data from the device anyway


which does not mean it's better unconditionally.
are above points clear?

The thing is, what blocks the config space solution?
Why admin vq is a must for live migration?
What's wrong in config space solution?
Shall we refactor everything in virtio-pci to use admin vq?


as long as you guys keep not hearing each other we will keep
seeing these flame wars. if you expect everyone on virtio-comment
to follow a 300 message thread you are imo very much mistaken.

I am sure I have not ignored any questions.
I am saying admin vq is problematic for live migration,
at least it doesn't work for nested, so why admin vq is a must for live migration? 







The general purpose of his proposal and mine are aligned: migrate virtio
devices.

Jason has ever proposed to collaborate, please allow me quote his proposal:

"
Let me repeat once again here for the possible steps to collaboration:

1) define virtqueue state, inflight descriptors in the section of
basic facility but not under the admin commands
2) define the dirty page tracking, device context/states in the
section of basic facility but not under the admin commands
3) define transport specific interfaces or admin commands to access them
"

I totally agree with his proposal.

Does this work for you Michael?

Thanks
Zhu Lingshan

I just doubt very much this will work.  What will "define" mean then -
not an interface, just a description in english? I think you
underestimate the difficulty of creating such definitions that
are robust and precise.

I think we can review the patch to correct the words.


Instead I suggest you define a way to submit admin commands that works
for nested and bare-metal (i.e. not admin vq, and not with sriov group
type). And work with Parav to make live migration admin commands work
reasonably will through this interface and with this type.

why admin commands are better than registers?

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