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

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

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.

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
>