Re: [PATCH RFC] vfio: Revise and update the migration uAPI description

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jan 25, 2022 at 03:55:31AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Saturday, January 15, 2022 3:35 AM
> > + *
> > + *   The peer to peer (P2P) quiescent state is intended to be a quiescent
> > + *   state for the device for the purposes of managing multiple devices
> > within
> > + *   a user context where peer-to-peer DMA between devices may be active.
> > The
> > + *   PRE_COPY_P2P and RUNNING_P2P states must prevent the device from
> > + *   initiating any new P2P DMA transactions. If the device can identify P2P
> > + *   transactions then it can stop only P2P DMA, otherwise it must stop all
> > + *   DMA.  The migration driver must complete any such outstanding
> > operations
> > + *   prior to completing the FSM arc into either P2P state.
> > + *
> 
> Now NDMA is renamed to P2P... but we did discuss the potential
> usage of using this state on devices which cannot stop DMA quickly 
> thus needs to drain pending page requests which further requires 
> running vCPUs if the fault is on guest I/O page table.

I think this needs to be fleshed out more before we can add it,
ideally along with a driver and some qemu implementation

It looks like the qemu part for this will not be so easy..

Jason