RE: [virtio-comment] [PATCH v1 1/8] admin: Add theory of operation for device migration

[Parav Pandit <parav@nvidia.com>]

> From: Parav Pandit
> Sent: Thursday, October 19, 2023 3:30 PM
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Thursday, October 19, 2023 3:23 PM
> >
> > On Thu, Oct 19, 2023 at 09:41:46AM +0000, Parav Pandit wrote:
> > >
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Thursday, October 19, 2023 3:03 PM
> > > > > > > With a capability you can discover it without poking at features.
> > > > > > >
> > > > > > It is discouraged by pci spec.
> > > > > > Pci caps for mostly doing very small init time sort of config.
> > > > > > Not to run frequent commands.
> > > > >
> > > > > Interesting. where in the spec exactly?
> > > >
> > > > And the reason I ask is because I'd like to understand the exact limitation.
> > > >
> > > > In any case, we should still maybe look for ways to separate it from config.
> > > > Maybe a capability points at a BAR and that is where we have this stuff?
> > > >
> > > Yes, I replied in your previous email, a similar suggestion.
> > > Lets first agree that it is drafted for non-passthrough mode.
> >
> > I could see passthrough use it too if there, why not.
> For three reasons:
> 1. it does not survive FLR
> 2. DMA occurs on the RID, A vm can directly attack to this programmed
> memory address in it if done without PASID 3. PASID is not available anytime
> soon in virtio 1.4-time frame on most common platforms.
And 4th, these new registers do not scale at VF.