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 09:41:40AM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Friday, November 17, 2023 3:08 PM
> > 
> > On Fri, Nov 17, 2023 at 09:14:21AM +0000, Parav Pandit wrote:
> > >
> > >
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Friday, November 17, 2023 2:16 PM In any case you can safely
> > > > assume that many users will have migration that takes seconds and
> > > > minutes.
> > >
> > > Strange, but ok. I don't see any problem with current method.
> > > 8MB is used for very large VM of 1TB takes minutes. Should be fine.
> > 
> > The problem is simple: vendors selling devices have no idea how large the VM
> > will be. So you have to over-provision for the max VM size.
> > If there was a way to instead allocate that in host memory, that would improve
> > on this.
> 
> Not sure what to over provision for max VM size.
> Vendor does not know how many vcpus will be needed. It is no different problem.
> 
> When the VM migration is started, the individual tracking range is supplied by the hypervisor to device.
> Device allocates necessary memory on this instruction.
> 
> When the VM with certain size is provisioned, the member device can be provisioned for the VM size.
> And if it cannot be provisioned, possibly this may not the right member device to use at that point in time.

For someone who keeps arguing against adding single bit registers
"because it does not scale" you seem very nonchalant about adding
8Mbytes.

I thought we have a nicely contained and orthogonal feature, so if it's
optional it's not a problem.

But with such costs and corner cases what exactly is the motivation for
the feature here?  Do you have a PoC showing how this works better than
e.g. shadow VQ?

Maybe IOMMU based and shadow VQ based tracking are the way to go
initially, and if there's a problem then we should add this later, on
top.

I really want us to finally make progress merging features and anything
that reduces scope initially is good for that.

-- 
MST