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Subject: Re: [virtio-dev] [PATCH] virtio-net: add mtu configuration field
On Fri, Dec 11, 2015 at 11:49:47AM -0500, Aaron Conole wrote: > Sometimes it is essential for libvirt to be able to configure MTU > on guest's NICs to a value different from 1500. > > The change adds a new field to configuration area of network > devices. It will be used to pass initial MTU from the device to > the driver, and to pass modified MTU from driver to the device > when a new MTU is assigned by the guest OS. > > In addition, in order to support backward and forward > compatibility, we introduce a new feature bit called > VIRTIO_NET_F_DEFAULT_MTU. > > Signed-off-by: Aaron Conole <aconole@redhat.com> > Cc: Victor Kaplansky <victork@redhat.com> > > --- > > This is an attempt at continuing the work done by Victor Kaplansky on > mtu negiotiation for virtio-net devices. It attempts to pick up from > https://lists.oasis-open.org/archives/virtio-dev/201508/msg00007.html > and is just a minor blurb from the first patch along with the 2nd patch > from the series, and some of the feedback integrated. > > trunk/content.tex | 20 +++++++++++++++++++++++++ > 1 file changed, 20 insertions(+) > > diff --git a/trunk/content.tex b/trunk/content.tex > --- a/trunk/content.tex (revision 551) > +++ b/trunk/content.tex (working copy) > @@ -3078,6 +3078,11 @@ > > \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control > channel. > + > +\item[VIRTIO_NET_F_MTU(24)] Negotiated MTU is supported. If > + offered by the device, device advises driver about initial > + MTU to be used. If negotiated, the driver uses \field{mtu} as > + an initial value. > \end{description} > > \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device / Feature bits / Feature bit requirements} > @@ -3132,11 +3137,15 @@ > and transmitq1\ldots transmitqN respectively) that can be configured once VIRTIO_NET_F_MQ > is negotiated. > > +The following driver-read-only field, \field{mtu} can be written by the driver > +after negotiation. > + > \begin{lstlisting} > struct virtio_net_config { > u8 mac[6]; > le16 status; > le16 max_virtqueue_pairs; > + le16 mtu; > }; > \end{lstlisting} > > @@ -3145,6 +3154,11 @@ > The device MUST set \field{max_virtqueue_pairs} to between 1 and 0x8000 inclusive, > if it offers VIRTIO_NET_F_MQ. > > +\devicenormative{\subsubsection}{Device configuration layout}{Device Types / Network Device / Device configuration layout} > + > +The device MUST set \field{mtu} to between 68 and 65535 inclusive, > +if it offers VIRTIO_NET_F_MTU. > + > \drivernormative{\subsubsection}{Device configuration layout}{Device Types / Network Device / Device configuration layout} > > A driver SHOULD negotiate VIRTIO_NET_F_MAC if the device offers it. > @@ -3157,6 +3171,12 @@ > assume the link is active, otherwise it SHOULD read the link status from > the bottom bit of \field{status}. > > +A driver SHOULD negotiate VIRTIO_NET_F_MTU if the device > +offers it. If the driver negotiates the VIRTIO_NET_F_MTU > +feature, the driver MAY use \field{mtu} as an initial value > +for MTU and > + the driver MAY report the value of MTU to > +\field{mtu} when MTU is modified by the guest. > + I think making mtu writeable like this will lead to inability to report host side mtu changes to guest. How about removing this part for now? > \subsubsection{Legacy Interface: Device configuration layout}\label{sec:Device Types / Network Device / Device configuration layout / Legacy Interface: Device configuration layout} > \label{sec:Device Types / Block Device / Feature bits / Device configuration layout / Legacy Interface: Device configuration layout} > When using the legacy interface, transitional devices and drivers > -- > 2.5.0 > > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org
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