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

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 <>
---
 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
--