Re: [PATCH 0/4] vl: Prioritize device realizations

[Daniel P . Berrangé]

On Wed, Oct 20, 2021 at 03:44:08PM +0200, David Hildenbrand wrote:
> On 18.08.21 21:42, Peter Xu wrote:
> > This is a long pending issue that we haven't fixed.  The issue is in QEMU we
> > have implicit device ordering requirement when realizing, otherwise some of 
> > the
> > device may not work properly.
> > 
> > The initial requirement comes from when vfio-pci starts to work with 
> > vIOMMUs.
> > To make sure vfio-pci will get the correct DMA address space, the vIOMMU 
> > device
> > needs to be created before vfio-pci otherwise vfio-pci will stop working 
> > when
> > the guest enables the vIOMMU and the device at the same time.
> > 
> > AFAIU Libvirt should have code that guarantees that.  For QEMU cmdline 
> > users,
> > they need to pay attention or things will stop working at some point.
> > 
> > Recently there's a growing and similar requirement on vDPA.  It's not a hard
> > requirement so far but vDPA has patches that try to workaround this issue.
> > 
> > This patchset allows us to realize the devices in the order that e.g. 
> > platform
> > devices will be created first (bus device, IOMMU, etc.), then the rest of
> > normal devices.  It's done simply by ordering the QemuOptsList of "device"
> > entries before realization.  The priority so far comes from migration
> > priorities which could be a little bit odd, but that's really about the same
> > problem and we can clean that part up in the future.
> > 
> > Libvirt can still keep its ordering for sure so old QEMU will still work,
> > however that won't be needed for new qemus after this patchset, so with the 
> > new
> > binary we should be able to specify qemu cmdline as wish on '-device'.
> > 
> > Logically this should also work for vDPA and the workaround code can be done
> > with more straightforward approaches.
> > 
> > Please review, thanks.
> 
> Hi Peter, looks like I have another use case:
> 
> vhost devices can heavily restrict the number of available memslots:
> e.g., upstream KVM ~64k, vhost-user usually 32 (!). With virtio-mem
> intending to make use of multiple memslots [1] and auto-detecting how
> many to use based on currently avilable memslots when plugging and
> realizing the virtio-mem device, this implies that realizing vhost
> devices (especially vhost-user device) after virtio-mem devices can
> similarly result in issues: when trying realization of the vhost device
> with restricted memslots, QEMU will bail out.
> 
> So similarly, we'd want to realize any vhost-* before any virtio-mem device.

Ordering virtio-mem vs vhost-* devices doesn't feel like a good
solution to this problem. eg if you start a guest with several
vhost-* devices, then virtio-mem auto-decides to use all/most
remaining memslots, we've now surely broken the abiltiy to then
hotplug more vhost-* devices at runtime by not leaving memslots
for them.

I think virtio-mem configuration needs to be stable in its memslot
usage regardless of how many other types of devices are present,
and not auto-adjust how many it consumes.

Regards,
Daniel
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