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



On 10/30/2023 12:17 PM, Parav Pandit wrote:



From: Zhu, Lingshan <lingshan.zhu@intel.com>
Sent: Monday, October 30, 2023 9:15 AM

On 10/26/2023 3:04 PM, Parav Pandit wrote:

From: Zhu, Lingshan <lingshan.zhu@intel.com>
Sent: Thursday, October 26, 2023 12:14 PM


On 10/24/2023 6:37 PM, Parav Pandit wrote:

From: Zhu, Lingshan <lingshan.zhu@intel.com>
Sent: Tuesday, October 24, 2023 4:00 PM

On 10/23/2023 6:14 PM, Parav Pandit wrote:

From: Zhu, Lingshan <lingshan.zhu@intel.com>
Sent: Monday, October 23, 2023 3:39 PM

On 10/20/2023 8:54 PM, Parav Pandit wrote:

From: Zhu, Lingshan <lingshan.zhu@intel.com>
Sent: Friday, October 20, 2023 3:01 PM

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]
https://lore.kernel.org/virtio-comment/870ace02-f99c-4582-932f
-b
d1
03
36


2dae9@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.


I will not respond to this comment anymore.

Ok if you choose not to respond.

For pass-through, I still recommend you to take a reference of
current virito-pci implementation, it works for pass-through, right?

What do you mean by current virtio-pci implementation?

current virito-pci works for pass-through

I still donât understand what is "current virtio-pci".
Do you mean qemu implementation of emulated virtio-pci or you mean

virtio-pci specification for passthrough?

What do you want me to refer to for passthrough? Please clarify.

you know guest vcpu and its vRC can not access host side devices,
and there must be a driver helping the pass-through use cases, like
vDPA and vfio

I am not sure how to corelate this answer to the question of
"virtio-pci for

passthrough".

:(

Today when a virtio-pci member device is passthrough to the guest
VM,

hypervisor is not involved in virtio interface such as config space,
cvq, data vq etc.

Do you agree?

You didnât respond yet to this question.
Can you please respond?

Not sure which question you refer to that not answered, Agree what?

What is listed above.


Please don't cut off the thread until
the issue closed.


I didnât cut off the thread. Please check your email client.

If you are asking whether hypervisor is involved in accessing virtio interfaces.
For passthrough, guest needs a host side help driver to access hardware, and
explained below.

Not an accurate answer. Please answer above.
Repeating the question again.
For passthrough device virtio interfaces such as common and device config space, cvq, data vqs, are NOT accessed by the hypervisor.
Do you agree?

Did you failed to process the answer?

Let me repeat again, for the last time.
The guest can not access any host side devices without a "pass-through helper driver".
And the helper driver could be considered as a part of the hypervisor, or the guest vCPU can not access the host side devices. 

For example, the path is hw-->vfio_pci-->qemu-->guest. IT IS NOT hw-->guest.

You can try to take a loot at how virtio-pci work for QEMU.
If you failed to understand this, then I don't see any necessities to discuss on this topic anymore. 



Can vCPU access host side device config space? It needs a
pass-through helper driver like vfio, right?

Right.
And if you are implying that, because generic pci config space is intercepted

hence, all virtio common and device specific things MUST BE ALWAYS
intercepted as well.

Then I do not agree with such derivation.

This is not only virtio, guest needs a helper to access host devices.

Does not make sense until you reply above.

see above



The main reasons are:
1. It breaks the future TDISP model

Not sure why you bring TDISP again, I thought we agree this is closed.


Not to include TDISP in current spec, but the mechanism/infrastructure built applies to the future mode as well.

then discuss in future.



How it break TDISP? Can you let guest driver access host device whiteout a host
side helper like VFIO?

Yes, once it is passthrough virtio interface is a secure channel.
In TDISP config space is still communicated via hypervisor and it contains all the data that is not critical.
Hence, there must not be any virtio registers to place in there.
In future if one discovers config space as problematic, one will find generic solution for all the pci devices, not just virtio.
interesting, how does guest vCPU access host side devices without a helper driver, even a secure channel? 

So do you mean even queue_enable should not be there??? really interesting.




And TDISP says you should not trust PF, thus you should not use admin vq on PF
for live migration.

There are few options which will evolve.
1. PF will be handed over to the TVM instead of hypervisor
2. PF aq communication will be encrypted hence not visible to hypervisor, also supported by PCI-SIG already
3. Some other options
Since this is the generic solution across virtio and non_virtio, we can rely on wider wisdom of PCI-SIG.

TDISP says don't trust PF anyway.



2. Without hypervisor getting involved, all the member device MMIO
space is accessible which follows the efficiency and equivalency
principle of Jason listed paper

I hope you are not implying to trap+emulate virtio interfaces (which is not

listed in the pci-spec) in hypervisor for member passthrough devices.
Do you agree mmap the bars(interfaces) without doing anything is also a type of
"trap and emulate"?

Certainly not.
Memory mapping enables guest to _directly_ communicate with the device without any VMEXITS.
In TDISP world this is also even secured already.
So no, it is not trap and emulate.

Interesting. do you know virtualization is built on "trap and emulate"?
and pass-through is a special case of "trap and emulate"

If you want to discuss TDISP, then TDISP secured device only accepts
TLP from the owner, that means it only support the special case, and
that is an limitation of TDISP.

But generally speaking, you can always choose to trap and emulate any fields
in the bar.

Why is TDISP related to current live migration proposal?
Why we are discussing this?



For scale, I already told you for many times that they are
per-device facilities. How can a per-device facility not scale?

Each VF device must implement new set of on-chip memory-based
registers

which demands more power, die area which does not scale
efficiently to thousands of VFs.
that can be fpga gates or SOC implementing new features, you
think that is a waste?

It is waste in hw, if there is a better approach possible to not
burn them as

gates and save on resources for rarely used items.
Is a new entry in MSIX table a waste of HW?

Not as must as existing MSI-X table entries which requires linear
amount of

on-chip memory.
anyway, even only one MSIX entry cost my HW resource than the amount
of new registers in my proposal.

Yes, this is why new MSI-X proposals are on table to improve, the first known

approach to me was from Intel using IMS.

Hence, virtio already learnt it seen in the Appendix to not keep adding non init

time registers.
non-sense to me, IMS still uses MSI

Clearly not.
May be you missed something.

IMS enables once to use non registers for the interrupt store unlike MSI/MSI-X.

Please see the commit log comment, snippet here about "queue memory".

        - The interrupt chip must provide the following optional callbacks
          when the irq_mask(), irq_unmask() and irq_write_msi_msg() callbacks
          cannot operate directly on hardware, e.g. in the case that the
          interrupt message store is in queue memory:

IRQ chips callback irq_write_msi_msg() has no such limitation to store in registers.
Please re-read my answer, I said: IMS uses MSI, I didn't say re-use PCI msi entries. 


Can I say implementing admin vq in SOC is a waste of cores?

Which cores in the SoC?
If it is on the PF, there is only handful of AQs for scale of N VFs.

I see you got the point anyway, new features cost extra resource

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.

Whatever is done in past is done, there is no way to change history.
An new non init time registers should not be placed in device
specific config

space as virtio spec has clear guideline on it for good.

Device context reading, dirty page address reading, changing vf
device modes,

all of these are clearly not a init time settings.

Hence, they do not belong to the registers.

reset vq? and you get it from Appendix B. Creating New Device
Types, are we implementing a new type of device???

I donât understand your question.
I replied the history of reset_vq.
Take good examples to follow, reset_vq clearly is not the one.

so again, we are not implementing new device type, so your citation
doesn't apply.

I disagree.
I am engineer to build practical systems considering limitations and
also advancements of the transport; while listening to other
industry efforts, I

am no from legal department.

Hence, Appendix B makes a sense to me to apply to the existing
device which

also has the section for "device improvements".
it titled as "new device", and I think this discussion is non-sense.
So if you want to fix this statement, works for me.

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