Re: [virtio-dev] [PATCH] virtio-net: add mtu configuration field

[Yan Vugenfirer <yvugenfi@redhat.com>]

> 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
>> 
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