[dpdk-users] Hyper-V multi-queue not working on Kernel v4.20

[Gareth Bradshaw <gareth@vectra.ai>]

Hi,

I'm developing a DPDK application for Hyper-V and would like to use Kernel v4.20 as it fixes a crash in previous kernels (per this pull request<https://github.com/torvalds/linux/commit/cdfa835c6e5e87d145f9f632b58843de97509f2b>) but multi-queue support seems to have broken in that kernel.  It works in kernels 4.17, 4.18 and 4.19.

Using testpmd with parameters "--forward-mode=txonly --nb-cores=3 --txq=3 --rxq=3" I get the following error "vmbus_uio_get_subchan(): invalid subchannel id 0".

Tracing the code a bit I find that it's trying to read /sys/bus/devices/<devid>/channels/21/subchannel_id but when I try to cat the same file I get the error "Invalid argument".  I've compared to previous kernel versions and see the following differences:

  *   When the VM boots, the device is bound to the kernel driver and there are 4 channels on both kernel versions.  It's a 4 core VM.
  *   When I bind the device to uio_hv_generic, kernel v.19.5 (and previous) has a single channel and it has a valid subchannel_id.  On kernel 4.20 the 4 channel folders remain but they are all invalid.
  *   When I start testpmd with 3 queues, on kernel v4.19.5 two additional channels are created, they're valid and testpmd works as expected.  On kernel 4.20 the 4 invalid channels remain invalid and testpmd gives the error I mentioned.

Any help getting this working would be greatly appreciated.

Thanks
Gareth