Re: [PATCH 4/4] vl: Prioritize realizations of devices

[Peter Xu <peterx@redhat.com>]

On Tue, Aug 24, 2021 at 06:24:27PM +0200, David Hildenbrand wrote:
> > > Not so much; here's the list of priorities and the devices using it:
> > > 
> > >         |--------------------+---------|
> > >         | priority           | devices |
> > >         |--------------------+---------|
> > >         | MIG_PRI_IOMMU      |       3 |
> > >         | MIG_PRI_PCI_BUS    |       7 |
> > >         | MIG_PRI_VIRTIO_MEM |       1 |
> > >         | MIG_PRI_GICV3_ITS  |       1 |
> > >         | MIG_PRI_GICV3      |       1 |
> > >         |--------------------+---------|
> > 
> > iommu is probably ok. I think virtio mem is ok too,
> > in that it is normally created by virtio-mem-pci ...
> 
> IIRC:
> 
> intel-iommu has to be created on the QEMU cmdline before creating
> virtio-mem-pci.
> 
> -device intel-iommu,caching-mode=on,intremap=on,device-iotlb=on \
> ...
> -device virtio-mem-pci,disable-legacy=on,disable-modern=off,iommu_platform=on,ats=on,id=vm0,...
> 
> Creating virtio-mem-pci will implicitly create virtio-mem. virtio-mem device
> state has to be migrated before migrating intel-iommu state.

Since we're at it.. frankly I didn't fully digest why virtio-mem needs to be
migrated before when reading commit 0fd7616e0f1171b.  It gives me the feeling
more like that virtio-mem has a ordering dependency with vfio-pci not
virtio-mem, but I could be wrong.

E.g., the IOMMU unit shouldn't be walking page table if without a device using
it, then it won't fail until the device walks it in region_add() hooks when
memory replay happens.

> 
> I do wonder if migration priorities are really what we want to reuse here. I
> guess it works right, but just by pure luck (because we ignore the implicit
> dependency regarding priorities)

Yep, that's probably not what we want.  I'll make it a new list of priorities
as I've stated in the other thread.

Thanks,

-- 
Peter Xu