Re: [virtio-dev] [PATCH v8] virtio-net: add Max MTU configuration field

["Michael S. Tsirkin" <mst@redhat.com>]

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.


> > ---
> > 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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