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Subject: Re: [virtio-dev] Re: [PATCH v2] virtio-net: add mtu configuration field
"Michael S. Tsirkin" <mst@redhat.com> writes: > 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. I think that was due to a previous patch which did not include a mechanism for reporting the guest->host changes. This includes such a feedback mechanism. However, I may have misunderstood Yan's previous email. Please advise, Yan. > 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? I am not very familiar with qemu-ga. If Yan says the new proposal will not work, then I will investigate that more thoroughly. >> --- >> 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 > > --------------------------------------------------------------------- > 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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