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

[David Hildenbrand]

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.

Do you have any updated version of this patchset? Thanks!

[1] 20211013103330.26869-1-david@redhat.com">https://lkml.kernel.org/r/20211013103330.26869-1-david@redhat.com


-- 
Thanks,

David / dhildenb