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: 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-bd103
> >>> 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.

> >
> >> 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.


> >
> >> 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.