[PATCH v2] virtio-net: Add an optional device control over the receive buffers length

[Vitaly Mireyno <vmireyno@marvell.com>]

This patch gives devices some level of control over the receive buffers length.
The driver declares the minimum receive buffer length, and the device requests max/min buffer length ratio.

v2 incorporates v1 comments.

Changes from v1:
 * min_rx_buf_len must be set before FEATURES_OK
 * Use a single feature bit VIRTIO_NET_F_RXBUF_LEN_CTRL for both, the min size and the ratio
 * min_rx_buf_len is defined as write-only for driver


Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>
---
 content.tex | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/content.tex b/content.tex
index d68cfaf..ab0d193 100644
--- a/content.tex
+++ b/content.tex
@@ -2815,6 +2815,10 @@ \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_RXBUF_LEN_CTRL(58)] Device requests to know the minimum
+    receive buffers length, and requests to limit the maximum/minimum receive
+    buffers length ratio. Driver declares the minimum receive buffers length.
+
 \item[VIRTIO_NET_F_GUEST_HDRLEN(59)] Driver can provide the exact \field{hdr_len}
     value. Device benefits from knowing the exact header length.
 
@@ -2861,8 +2865,8 @@ \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.
@@ -2882,12 +2886,22 @@ \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 write-only for driver field \field{min_rx_buf_len} only exists if
+VIRTIO_NET_F_RXBUF_LEN_CTRL is set. This field specifies the minimum receive
+buffers length.
+
+The driver-read-only field \field{rx_buf_len_ratio} only exists if
+VIRTIO_NET_F_RXBUF_LEN_CTRL is set. This field specifies the maximum/minimum receive
+buffers length ratio. The value '0' indicates unrestricted ratio.
+
 \begin{lstlisting}
 struct virtio_net_config {
         u8 mac[6];
         le16 status;
         le16 max_virtqueue_pairs;
         le16 mtu;
+        le32 min_rx_buf_len;
+        le16 rx_buf_len_ratio;
 };
 \end{lstlisting}
 
@@ -2916,6 +2930,13 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
 If the driver negotiates the VIRTIO_NET_F_STANDBY feature, the device MAY act
 as a standby device for a primary device with the same MAC address.
 
+A driver SHOULD accept the VIRTIO_NET_F_RXBUF_LEN_CTRL feature if offered.
+
+If VIRTIO_NET_F_RXBUF_LEN_CTRL feature has been negotiated, the driver MUST
+set \field{min_rx_buf_len} before setting the FEATURES_OK status bit.
+
+A driver MUST NOT modify \field{min_rx_buf_len} once it has been set.
+
 \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.
@@ -3281,6 +3302,13 @@ \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_RXBUF_LEN_CTRL feature has been negotiated, the driver
+MUST initialize all receive virtqueue descriptors \field{len} field with
+the value greater than or equal to the value configured in the
+\field{min_rx_buf_len} device configuration field, and allocate receive
+buffers accordingly. If \field{rx_buf_len_ratio} > 0, then the \field{len} value
+must also be less than or equal to (\field{min_rx_buf_len} * \field{rx_buf_len_ratio}).
+
 \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
--