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

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

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.

> 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.

-- 
MST