Re: [virtio-comment] Re: [PATCH v3 6/8] admin: Add theory of operation for write recording commands

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 17, 2023 at 10:03:47AM +0000, Parav Pandit wrote:
> 
> 
> > From: Zhu, Lingshan <lingshan.zhu@intel.com>
> > Sent: Friday, November 17, 2023 3:30 PM
> > 
> > On 11/16/2023 7:59 PM, Michael S. Tsirkin wrote:
> > > On Thu, Nov 16, 2023 at 06:28:07PM +0800, Zhu, Lingshan wrote:
> > >>
> > >> On 11/16/2023 1:51 PM, Michael S. Tsirkin wrote:
> > >>> On Thu, Nov 16, 2023 at 05:29:54AM +0000, Parav Pandit wrote:
> > >>>> We should expose a limit of the device in the proposed
> > WRITE_RECORD_CAP_QUERY command, that how much range it can track.
> > >>>> So that future provisioning framework can use it.
> > >>>>
> > >>>> I will cover this in v5 early next week.
> > >>> I do worry about how this can even work though. If you want a
> > >>> generic device you do not get to dictate how much memory VM has.
> > >>>
> > >>> Aren't we talking bit per page? With 1TByte of memory to track ->
> > >>> 256Gbit -> 32Gbit -> 8Gbyte per VF?
> > >>>
> > >>> And you happily say "we'll address this in the future" while at the
> > >>> same time fighting tooth and nail against adding single bit status
> > >>> registers because scalability?
> > >>>
> > >>>
> > >>> I have a feeling doing this completely theoretical like this is problematic.
> > >>> Maybe you have it all laid out neatly in your head but I suspect not
> > >>> all of TC can picture it clearly enough based just on spec text.
> > >>>
> > >>> We do sometimes ask for POC implementation in linux / qemu to
> > >>> demonstrate how things work before merging code. We skipped this for
> > >>> admin things so far but I think it's a good idea to start doing it
> > >>> here.
> > >>>
> > >>> What makes me pause a bit before saying please do a PoC is all the
> > >>> opposition that seems to exist to even using admin commands in the
> > >>> 1st place. I think once we finally stop arguing about whether to use
> > >>> admin commands at all then a PoC will be needed before merging.
> > >> We have POR productions that implemented the approach in my series.
> > >> They are multiple generations of productions in market and running in
> > >> customers data centers for years.
> > >>
> > >> Back to 2019 when we start working on vDPA, we have sent some samples
> > >> of production(e.g., Cascade Glacier) and the datasheet, you can find
> > >> live migration facilities there, includes suspend, vq state and other
> > >> features.
> > >>
> > >> And there is an reference in DPDK live migration, I have provided
> > >> this page
> > >> before:
> > >> https://doc.dpdk.org/guides-21.11/vdpadevs/ifc.html, it has been
> > >> working for long long time.
> > >>
> > >> So if we let the facts speak, if we want to see if the proposal is
> > >> proven to work, I would
> > >> say: They are POR for years, customers already deployed them for years.
> > > And I guess what you are trying to say is that this patchset we are
> > > reviewing here should be help to the same standard and there should be
> > > a PoC? Sounds reasonable.
> > Yes and the in-marketing productions are POR, the series just improves the
> > design, for example, our series also use registers to track vq state, but
> > improvements than CG or BSC. So I think they are proven to work.
> 
> If you prefer to go the route of POR and production and proven documents etc, there is ton of it of multiple types of products I can dump here with open-source code and documentation and more.
> Let me know what you would like to see.
> 
> Michael has requested some performance comparisons, not all are ready to share yet.
> Some are present that I will share in coming weeks.
> 
> And all the vdpa dpdk you published does not have basic CVQ support when I last looked at it.
> Do you know when was it added?

It's good enough for PoC I think, CVQ or not.
The problem with CVQ generally, is that VDPA wants to shadow CVQ it at all times
because it wants to decode and cache the content. But this problem has nothing
to do with dirty tracking even though it also mentions "shadow":
if device can report it's state then there's no need to shadow CVQ.

> > >
> > >> For dirty page tracking, I see you want both platform IOMMU tracking
> > >> and shadow vqs, I am totally fine with this idea. And I think maybe
> > >> we should merge the basic features first, and dirty page tracking
> > >> should be the second step.
> > >>
> > >> Thanks
> > > Parav wants to add an option of on-device tracking. Which also seems
> > > fine. I think it should be optional though because shadow and IOMMU
> > > options exist.
> > I agree, the vendor can choose to implement their own facility as a backup.
> > >
>