Re: [PATCH 01/18] vfio/migration: Add VFIO migration pre-copy support

[Jason Gunthorpe]

On Tue, Jan 31, 2023 at 03:43:01PM -0700, Alex Williamson wrote:

> How does this affect our path towards supported migration?  I'm
> thinking about a user experience where QEMU supports migration if
> device A OR device B are attached, but not devices A and B attached to
> the same VM.  We might have a device C where QEMU supports migration
> with B AND C, but not A AND C, nor A AND B AND C.  This would be the
> case if device B and device C both supported P2P states, but device A
> did not. The user has no observability of this feature, so all of this
> looks effectively random to the user.

I think qemu should just log if it encounters a device without P2P
support.

> Even in the single device case, we need to make an assumption that a
> device that does not support P2P migration states (or when QEMU doesn't
> make use of P2P states) cannot be a DMA target, or otherwise have its
> MMIO space accessed while in a STOP state.  Can we guarantee that when
> other devices have not yet transitioned to STOP?

You mean the software devices created by qemu?

> We could disable the direct map MemoryRegions when we move to a STOP
> state, which would give QEMU visibility to those accesses, but besides
> pulling an abort should such an access occur, could we queue them in
> software, add them to the migration stream, and replay them after the
> device moves to the RUNNING state?  We'd need to account for the lack of
> RESUMING_P2P states as well, trapping and queue accesses from devices
> already RUNNING to those still in RESUMING (not _P2P).

I think any internal SW devices should just fail all accesses to the
P2P space, all the time.

qemu simply acts like a real system that doesn't support P2P.

IMHO this is generally the way forward to do multi-device as well,
remove the MMIO from all the address maps: VFIO, SW access, all of
them. Nothing can touch MMIO except for the vCPU.

> This all looks complicated.  Is it better to start with requiring P2P
> state support?  Thanks,

People have built HW without it, so I don't see this as so good..

Jason