Re: [RFC 15/18] vfio/iommufd: Implement iommufd backend

[Alex Williamson]

On Tue, 26 Apr 2022 11:11:56 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Tue, Apr 26, 2022 at 10:08:30PM +0800, Yi Liu wrote:
> 
> > > I think it is strange that the allowed DMA a guest can do depends on
> > > the order how devices are plugged into the guest, and varys from
> > > device to device?
> > > 
> > > IMHO it would be nicer if qemu would be able to read the new reserved
> > > regions and unmap the conflicts before hot plugging the new device. We
> > > don't have a kernel API to do this, maybe we should have one?  
> > 
> > For userspace drivers, it is fine to do it. For QEMU, it's not quite easy
> > since the IOVA is GPA which is determined per the e820 table.  
> 
> Sure, that is why I said we may need a new API to get this data back
> so userspace can fix the address map before attempting to attach the
> new device. Currently that is not possible at all, the device attach
> fails and userspace has no way to learn what addresses are causing
> problems.

We have APIs to get the IOVA ranges, both with legacy vfio and the
iommufd RFC, QEMU could compare these, but deciding to remove an
existing mapping is not something to be done lightly.  We must be
absolutely certain that there is no DMA to that range before doing so.
 
> > > eg currently I see the log messages that it is passing P2P BAR memory
> > > into iommufd map, this should be prevented inside qemu because it is
> > > not reliable right now if iommufd will correctly reject it.  
> > 
> > yeah. qemu can filter the P2P BAR mapping and just stop it in qemu. We
> > haven't added it as it is something you will add in future. so didn't
> > add it in this RFC. :-) Please let me know if it feels better to filter
> > it from today.  
> 
> I currently hope it will use a different map API entirely and not rely
> on discovering the P2P via the VMA. eg using a DMABUF FD or something.
> 
> So blocking it in qemu feels like the right thing to do.

Wait a sec, so legacy vfio supports p2p between devices, which has a
least a couple known use cases, primarily involving GPUs for at least
one of the peers, and we're not going to make equivalent support a
feature requirement for iommufd?  This would entirely fracture the
notion that iommufd is a direct replacement and upgrade from legacy
vfio and make a transparent transition for libvirt managed VMs
impossible.  Let's reconsider.  Thanks,

Alex