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Subject: Re: [virtio-dev] [PATCH v8] virtio-net: add Max MTU configuration field
"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Sun, Sep 04, 2016 at 08:50:57PM +0300, Michael S. Tsirkin wrote:
>> On Sun, Sep 04, 2016 at 10:31:09AM -0400, Aaron Conole wrote:
>> > It is helpful for a host to indicate an advisory MTU value to be set
>> > on guest's NICs other than the assumed 1500 byte value.
>>
>> This part of the description is wrong now.
>> It is no longer advisory and it should say "to indicate its MTU value".
>>
>> > This helps in
>> > situations where the host network is using Jumbo Frames, or aiding in
>> > PMTU discovery by configuring a homogenous network.
>>
>> Maybe add
>> ... It is also helpful for sizing receive buffers correctly.
>>
>> > The change adds a new field to configuration area of network
>> > devices. It will be used to pass a maximum MTU from the device to
>> > the driver. This will be used by the driver as a maximum value for
>> > packet sizes during transmission, without segmentation offloading.
>> >
>> > In addition, in order to support backward and forward compatibility,
>> > we introduce a new feature bit called VIRTIO_NET_F_MTU.
>> >
>> > Signed-off-by: Aaron Conole <aconole@redhat.com>
>> > Cc: Victor Kaplansky <victork@redhat.com>
>>
>> Acked-by: Michael S. Tsirkin <mst@redhat.com>
>>
>
> I created an issue to track this.
> Pls post with a fixed commit log and add
>
> VIRTIO-152
>
> on a line by itself in the log.
>
Thanks so much for the feedback, Michael. I have applied the changes
you requested, and added your ACK.
-Aaron
>> > ---
>> > 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.
>> >
>> > v3:
>> > Converted to just support initial MTU. Guest->host and Host->guest MTU
>> > changes are outside the scope of this change.
>> >
>> > v4:
>> > Removed references to 'initial', since that condition cannot be tested.
>> > Simply state that if the driver will use the mtu field, it must
>> > negotiate the feature bit, and if not, it must not.
>> >
>> > v5:
>> > After feedback from Michael S. Tsirkin
>> >
>> > v6:
>> > Bit has to change to bit 25 due to an undocumented bit 24 being taken.
>> >
>> > v7:
>> > Partial rewrite with feedback from MST. Additional conformance statements
>> > added.
>> >
>> > v8:
>> > Clarified the new conformance statements.
>> >
>> > Previous series:
>> > https://lists.oasis-open.org/archives/virtio-dev/201608/msg00056.html
>> >
>> > Note: should this proposal be accepted and approved, one or more
>> > claims disclosed to the TC admin and listed on the Virtio TC
>> > IPR page https://www.oasis-open.org/committees/virtio/ipr.php
>> > might become Essential Claims.
>> >
>> >
>> > content.tex | 34 ++++++++++++++++++++++++++++++++++
>> > 1 file changed, 34 insertions(+)
>> >
>> > diff --git a/content.tex b/content.tex
>> > index 4b45678..b90cbad 100644
>> > --- a/content.tex
>> > +++ b/content.tex
>> > @@ -3049,6 +3049,14 @@ features.
>> > \item[VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2)] Control channel offloads
>> > reconfiguration support.
>> >
>> > +\item[VIRTIO_NET_F_MTU(3)] Maximum negotiated MTU is supported. If
>> > + offered by the device, device advises driver about the value of
>> > + MTU to be used. If negotiated, the driver uses \field{mtu} as
>> > + the maximum MTU value supplied to the operating system.
>> > +
>> > + Note: many operating systems override the MTU value provided by the
>> > + driver.
>> > +
>> > \item[VIRTIO_NET_F_MAC (5)] Device has given MAC address.
>> >
>> > \item[VIRTIO_NET_F_GUEST_TSO4 (7)] Driver can receive TSOv4.
>> > @@ -3140,11 +3148,16 @@ 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 maximum MTU for the driver to
>> > +use.
>> > +
>> > \begin{lstlisting}
>> > struct virtio_net_config {
>> > u8 mac[6];
>> > le16 status;
>> > le16 max_virtqueue_pairs;
>> > + le16 mtu;
>> > };
>> > \end{lstlisting}
>> >
>> > @@ -3153,6 +3166,18 @@ 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.
>> >
>> > +The device MUST set \field{mtu} to between 68 and 65535 inclusive,
>> > +if it offers VIRTIO_NET_F_MTU.
>> > +
>> > +The device MUST NOT modify \field{mtu} once it has been set.
>> > +
>> > +The device MUST NOT pass received packets that exceed \field{mtu} size
>> > +with \field{gso_type} NONE or ECN if it offers VIRTIO_NET_F_MTU.
>> > +
>> > +The device MUST forward transmitted packets of up to MTU size with
>> > +\field{gso_type} NONE or ECN, and do so without fragmentation, 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.
>> > @@ -3165,6 +3190,15 @@ 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}.
>> >
>> > +If the device offers VIRTIO_NET_F_MTU, a driver MUST supply enough receive
>> > +buffers of size \field{mtu} to be able to receive at least one receive
>> > +packet with \field{gso_type} NONE or ECN.
>> > +
>> > +A driver SHOULD negotiate VIRTIO_NET_F_MTU if the device offers it.
>> > +
>> > +If the device offers VIRTIO_NET_F_MTU, a driver MUST NOT transmit packets of
>> > +size exceeding the value of \field{mtu} with \field{gso_type} NONE or ECN
>> > +
>> > \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
>> > --
>> > 2.7.4
>> >
>> >
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