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Subject: [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}
 
@@ -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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