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

[Parav Pandit <parav@nvidia.com>]

> 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?

> 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.

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

> >
> >>>>>> 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.

> >
> >> 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.