[Bug 207446] Hang bringing up vtnet(4) on >8 cpu GCE VMs

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207446

            Bug ID: 207446
           Summary: Hang bringing up vtnet(4) on >8 cpu GCE VMs
           Product: Base System
           Version: 10.3-BETA2
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: wac at google.com
                CC: freebsd-amd64 at FreeBSD.org
                CC: freebsd-amd64 at FreeBSD.org

Created attachment 167336
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=167336&action=edit
patch to sys/dev/virtio/network/if_vtnetvar.h

FreeBSD on Google Compute Engine currently requires "hw.vtnet.mq_disable=1" on
virtual machines of more than 8 CPUs to avoid a hang when bringing up the vtnet
interface.

I have prepared a patch which raises the VTNET_MAX_QUEUE_PAIRS to 64 which
solves this hang on GCE VMs with 64 or fewer CPUs. 

With this patch, the release engineer should be able to remove the
hw.vtnet.mq_disable line from the /boot/loader.conf that is placed in images
intended for Google Compute Engine and users should see the benefits of
multiqueue as a result.

The current limit of 8 is in CURRENT as well as earlier release branches (e.g.
stable/10).

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