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Subject: Re: [virtio-dev] [PATCH] virtio-net: add mtu configuration field
> On 15 Dec 2015, at 12:24, Michael S. Tsirkin <mst@redhat.com> wrote: > > 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? Actually I am OK with this part. Windows will change driver configuration -> driver will report MTU to the host. We don’t have the way to support host report of MTU change to the guest on Windows. > >> \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 > > --------------------------------------------------------------------- > 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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