[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 12:51:40AM -0500, 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?

Ugh. Actually of course:
With 1TByte of memory to track -> 256Mbit -> 32Mbit -> 8Mbyte per VF

8Gbyte per *PF* with 1K VFs.



> 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.
> 
> 
> -- 
> MST


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