[Bug 213201] [bhyve] virtio-net requires more than one descriptor per tx operation

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=213201

            Bug ID: 213201
           Summary: [bhyve] virtio-net requires more than one descriptor
                    per tx operation
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: misc
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: ricarkol at gmail.com

Hello,

We noticed that the Tx path for bhyve virtio-net devices
(pci_virtio_net.c:pci_vtnet_proctx) assumes that all Tx'es will have a first
descriptor with the header alone
(https://svnweb.freebsd.org/base/head/usr.sbin/bhyve/pci_virtio_net.c?view=markup#l609):

        sc->pci_vtnet_tx(sc, &iov[1], n - 1, plen); // Skips the header
descriptor

The issue is that the virtio 1.0 specification allows for having the net header
and the data in a single descriptor. See section 5.1.6.2 of the virtio spec
(http://docs.oasis-open.org/virtio/virtio/v1.0/cs04/virtio-v1.0-cs04.html#x1-1680006).

Thanks,
Ricardo

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