Re: [virtio-comment] [PATCH] virtio-net: Add equal-sized receive buffers feature

[Yuri Benditovich <ybendito@redhat.com>]

----- Original Message ----- 

> From: "Vitaly Mireyno" <vmireyno@marvell.com>
> To: virtio-comment@lists.oasis-open.org
> Cc: "Michael S. Tsirkin" <mst@redhat.com>, "Jason Wang" <jasowang@redhat.com>
> Sent: Thursday, October 31, 2019 11:23:52 AM
> Subject: [virtio-comment] [PATCH] virtio-net: Add equal-sized receive buffers
> feature

> Some devices benefit from working with receive buffers of a predefined
> constant length.
> Add a feature for drivers that allocate equal-sized receive buffers, and
> devices that benefit from predefined receive buffers length.

> Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>
> ---
> content.tex | 29 +++++++++++++++++++++++++++--
> 1 file changed, 27 insertions(+), 2 deletions(-)

> diff --git a/content.tex b/content.tex
> index b1ea9b9..c9e67c8 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -2811,6 +2811,10 @@ \subsection{Feature bits}\label{sec:Device Types /
> Network Device / Feature bits
> \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
> channel.

> +\item[VIRTIO_NET_F_CONST_RXBUF_LEN(58)] Driver allocates all receive buffers
> + of the same constant length. Device benefits from working with
> + receive buffers of equal length.
> +
> \item[VIRTIO_NET_F_RSC_EXT(61)] Device can process duplicated ACKs
> and report number of coalesced segments and duplicated ACKs

> @@ -2854,8 +2858,8 @@ \subsubsection{Legacy Interface: Feature
> bits}\label{sec:Device Types / Network
> \subsection{Device configuration layout}\label{sec:Device Types / Network
> Device / Device configuration layout}
> \label{sec:Device Types / Block Device / Feature bits / Device configuration
> layout}

> -Three driver-read-only configuration fields are currently defined. The
> \field{mac} address field
> -always exists (though is only valid if VIRTIO_NET_F_MAC is set), and
> +The driver-read-only \field{mac} address field always exists
> +(though is only valid if VIRTIO_NET_F_MAC is set), and
> \field{status} only exists if VIRTIO_NET_F_STATUS is set. Two
> read-only bits (for the driver) are currently defined for the status field:
> VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.
> @@ -2875,12 +2879,17 @@ \subsection{Device configuration
> layout}\label{sec:Device Types / Network Device
> VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the driver
> to
> use.

> +The device-read-only field \field{rx_buf_len} only exists if
> +VIRTIO_NET_F_CONST_RXBUF_LEN is negotiated. This field specifies the receive
> +buffers length.
> +
> \begin{lstlisting}
> struct virtio_net_config {
> u8 mac[6];
> le16 status;
> le16 max_virtqueue_pairs;
> le16 mtu;
> + le32 rx_buf_len;
> };
> \end{lstlisting}

Please see 
https://github.com/torvalds/linux/blob/master/include/uapi/linux/virtio_net.h
Note that there are already 2 additional fields not covered by the spec at the moment.

> @@ -2933,6 +2942,13 @@ \subsection{Device configuration
> layout}\label{sec:Device Types / Network Device

> A driver SHOULD negotiate the VIRTIO_NET_F_STANDBY feature if the device
> offers it.

> +A driver SHOULD accept the VIRTIO_NET_F_CONST_RXBUF_LEN feature if offered.
> +
> +If VIRTIO_NET_F_CONST_RXBUF_LEN feature has been negotiated, the driver MUST
> +set \field{rx_buf_len}.
> +
> +A driver MUST NOT modify \field{rx_buf_len} once it has been set.
> +
> \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
> @@ -3241,6 +3257,11 @@ \subsubsection{Setting Up Receive
> Buffers}\label{sec:Device Types / Network Devi
> If VIRTIO_NET_F_MQ is negotiated, each of receiveq1\ldots receiveqN
> that will be used SHOULD be populated with receive buffers.

> +If VIRTIO_NET_F_CONST_RXBUF_LEN feature has been negotiated, the driver
> +MUST initialize all receive virtqueue descriptors \field{len} field with
> +the value configured in \field{rx_buf_len} device configuration field,
> +and allocate receive buffers accordingly.
> +
> \devicenormative{\paragraph}{Setting Up Receive Buffers}{Device Types /
> Network Device / Device Operation / Setting Up Receive Buffers}

> The device MUST set \field{num_buffers} to the number of descriptors used to
> @@ -3396,6 +3417,10 @@ \subsubsection{Processing of Incoming
> Packets}\label{sec:Device Types / Network
> checksum (in case of multiple encapsulated protocols, one level
> of checksums is validated).

> +If VIRTIO_NET_F_CONST_RXBUF_LEN has been negotiated, the device MAY
> +use \field{rx_buf_len} as a buffer length (instead of reading it from
> +virtqueue descriptor \field{len} field).
> +
> \drivernormative{\paragraph}{Processing of Incoming
> Packets}{Device Types / Network Device / Device Operation /
> Processing of Incoming Packets}
> --

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