[virtio-comment] content: RSC extension support for virtio-net

[Yuri Benditovich <yuri.benditovich@daynix.com>]

Allocates feature bit for extended RSC (receive
side coalescing) support. If the device supports
this feature, it is able to process duplicated
ACKs and report number of coalesced TCP segments
amd number of received duplicated ACKs.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/20

Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
---
 content.tex | 39 ++++++++++++++++++++++++++++++---------
 1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/content.tex b/content.tex
index be18234..17d83af 100644
--- a/content.tex
+++ b/content.tex
@@ -2525,6 +2525,10 @@ features.
 
 \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
     channel.
+
+\item[VIRTIO_NET_F_RSC_EXT(38)] Device can process duplicated ACKs
+    and report number of coalesced segments and duplicated ACKs
+
 \end{description}
 
 \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device / Feature bits / Feature bit requirements}
@@ -2548,6 +2552,7 @@ Some networking feature bits require other networking feature bits
 \item[VIRTIO_NET_F_GUEST_ANNOUNCE] Requires VIRTIO_NET_F_CTRL_VQ.
 \item[VIRTIO_NET_F_MQ] Requires VIRTIO_NET_F_CTRL_VQ.
 \item[VIRTIO_NET_F_CTRL_MAC_ADDR] Requires VIRTIO_NET_F_CTRL_VQ.
+\item[VIRTIO_NET_F_RSC_EXT] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
 \end{description}
 
 \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Network Device / Feature bits / Legacy Interface: Feature bits}
@@ -2716,6 +2721,7 @@ itself is preceded by a header:
 struct virtio_net_hdr {
 #define VIRTIO_NET_HDR_F_NEEDS_CSUM    1
 #define VIRTIO_NET_HDR_F_DATA_VALID    2
+#define VIRTIO_NET_HDR_F_RSC_INFO      4
         u8 flags;
 #define VIRTIO_NET_HDR_GSO_NONE        0
 #define VIRTIO_NET_HDR_GSO_TCPV4       1
@@ -2879,8 +2885,8 @@ been negotiated, the driver SHOULD set \field{hdr_len} to a value
 not less than the length of the headers, including the transport
 header.
 
-The driver MUST NOT set the VIRTIO_NET_HDR_F_DATA_VALID bit in
-\field{flags}.
+The driver MUST NOT set the VIRTIO_NET_HDR_F_DATA_VALID and
+VIRTIO_NET_HDR_F_RSC_INFO bits in \field{flags}.
 
 \devicenormative{\paragraph}{Packet Transmission}{Device Types / Network Device / Device Operation / Packet Transmission}
 The device MUST ignore \field{flag} bits that it does not recognize.
@@ -2996,18 +3002,25 @@ transmit segmentation offloading and ECN features, as described
 in \ref{sec:Device Types / Network Device / Device Operation /
 Packet Transmission}:
 \begin{enumerate}
+\item If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were
+  negotiated, then \field{gso_type} MAY be something other than
+  VIRTIO_NET_HDR_GSO_NONE, and \field{gso_size} field indicates the
+  desired MSS (see Packet Transmission point 2).
+\item If the VIRTIO_NET_F_RSC_EXT option was negotiated (this
+  implies one of VIRTIO_NET_F_GUEST_TSO4, TSO6), the
+  device processes also duplicated ACK segments, reports
+  number of coalesced TCP segments in \field{csum_start} field and
+  number of duplicated ACK segments in \field{csum_offset} field
+  and sets bit VIRTIO_NET_HDR_F_RSC_INFO in \field{flags}.
 \item If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the
   VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} can be
   set: if so, the packet checksum at offset \field{csum_offset}
   from \field{csum_start} and any preceding checksums
   have been validated.  The checksum on the packet is incomplete and
-  \field{csum_start} and \field{csum_offset} indicate how to calculate
-  it (see Packet Transmission point 1).
+  if bit VIRTIO_NET_HDR_F_RSC_INFO is not set in \field{flags},
+  then \field{csum_start} and \field{csum_offset} indicate how to calculate it
+  (see Packet Transmission point 1).
 
-\item If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were
-  negotiated, then \field{gso_type} MAY be something other than
-  VIRTIO_NET_HDR_GSO_NONE, and \field{gso_size} field indicates the
-  desired MSS (see Packet Transmission point 2).
 \end{enumerate}
 
 \devicenormative{\paragraph}{Processing of Incoming Packets}{Device Types / Network Device / Device Operation / Processing of Incoming Packets}
@@ -3072,6 +3085,13 @@ VIRTIO_NET_HDR_GSO_NONE.
 If \field{gso_type} differs from VIRTIO_NET_HDR_GSO_NONE, then
 the device MUST also set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in
 \field{flags} MUST set \field{gso_size} to indicate the desired MSS.
+If VIRTIO_NET_F_RSC_EXT was negotiated, the device MUST also
+set VIRTIO_NET_HDR_F_RSC_INFO bit in \field{flags},
+set \field{csum_start} to number of coalesced TCP segments and
+set \field{csum_offset} to number of received duplicated ACK segments.
+
+If VIRTIO_NET_F_RSC_EXT was not negotiated, the device MUST
+not set VIRTIO_NET_HDR_F_RSC_INFO bit in \field{flags}.
 
 If one of the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options have
 been negotiated, the device SHOULD set \field{hdr_len} to a value
@@ -3090,7 +3110,8 @@ Processing of Incoming Packets}
 
 The driver MUST ignore \field{flag} bits that it does not recognize.
 
-If VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} is not set, the
+If VIRTIO_NET_HDR_F_NEEDS_CSUM bit in \field{flags} is not set or
+if VIRTIO_NET_HDR_F_RSC_INFO bit \field{flags} is set, the
 driver MUST NOT use the \field{csum_start} and \field{csum_offset}.
 
 If one of the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options have
-- 
2.17.0