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

["Zhu, Lingshan" <lingshan.zhu@intel.com>]

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-bd1
> > > > > > > 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?
Can vCPU access host side device config space? It needs a pass-through 
helper driver like vfio, right?
> > > > > > 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.
> > 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.