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Subject: Re: [PATCH v2] virtio-net: add mtu configuration field
On Fri, Jan 15, 2016 at 04:28:30PM -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 > or from device to driver for both host-guest and guest-host > interaction. > > 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> Since apparently windows guests can't support such mtu negotiation in the driver, we'd have to implement this in a guest agent for these guests. It thus seems advantageous to always have it in a guest agent, so that libvirt can change it for all guests in the same way, consistently. Do you agree to the above? If yes, why do we need a new device field? > --- > v1: > 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. > > v2: > Rephrase and provide a mechanism for guest->host and host->guest > communication through a driver read-only and driver write-only field. > > content.tex | 31 +++++++++++++++++++++++++++++++ > 1 file changed, 31 insertions(+) > > diff --git a/content.tex b/content.tex > index d989d98..b855691 100644 > --- a/content.tex > +++ b/content.tex > @@ -3078,6 +3078,11 @@ features. > > \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 +3137,21 @@ of each of transmit and receive virtqueues (receiveq1\ldots receiveqN > and transmitq1\ldots transmitqN respectively) that can be configured once VIRTIO_NET_F_MQ > is negotiated. > > +The following driver-read-only field, \field{mtu} only exists if > +VIRTIO_NET_F_MTU is set. This field specifies the MTU that the device expects > +the driver to honor. > + > +The following driver-write-only field, \field{mtu_requested} only exists if > +VIRTIO_NET_F_MTU is set. This field will be written by the driver when it wants > +to request an MTU change from the device. > + > \begin{lstlisting} > struct virtio_net_config { > u8 mac[6]; > le16 status; > le16 max_virtqueue_pairs; > + le16 mtu; > + le16 mtu_requested; > }; > \end{lstlisting} > > @@ -3145,6 +3160,18 @@ struct virtio_net_config { > 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. > + > +Any time the device modifies \field{mtu}, it MUST send a configuration change > +notification. > + > +The driver MAY set \field{mtu_requested} to between 68 and 65535 inclusive, > +if the device offers VIRTIO_NET_F_MTU. Any time the driver modifies > +\field{mtu_requested}, it MUST also send a configuration change notification. > + > \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 +3184,10 @@ If the driver does not negotiate the VIRTIO_NET_F_STATUS feature, it SHOULD > 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 MUST use \field{mtu} > +as the value for MTU. > + > \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
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