[RFC PATCH] virtio-spec: Introduce new Receive-Segment-Coalescing feature for guest

[Wei Xu <wexu@redhat.com>]

Hi TCers,
Recently i'm working on a feature named as Receive-Segment-Coalescing 
for userspace virtio-net, which is mainly aimed to support WHQL test 
case for windows guest, the below link is the detailed information from 
microsoft.

https://msdn.microsoft.com/en-us/library/windows/hardware/jj853323%28v=vs.85%29.aspx

A new feature bit and a few new fields in virtio network header are 
introduced to achieve this goal, please consider the request.

Index: content.tex
===================================================================
--- content.tex    (revision 569)
+++ content.tex    (working copy)
@@ -3086,6 +3086,11 @@

 \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
     channel.
+
+\item[VIRTIO_NET_F_GUEST_RSC4 (24)] Driver can receive coalesced ipv4 
tcp packets.
+
+\item[VIRTIO_NET_F_GUEST_RSC6 (25)] Driver can receive coalesced ipv6 
tcp packets.
+
 \end{description}

 \subsubsection{Feature bit requirements}\label{sec:Device Types / 
Network Device / Feature bits / Feature bit requirements}
@@ -3247,8 +3252,11 @@
         u8 flags;
 #define VIRTIO_NET_HDR_GSO_NONE        0
 #define VIRTIO_NET_HDR_GSO_TCPV4       1
-#define VIRTIO_NET_HDR_GSO_UDP         3
-#define VIRTIO_NET_HDR_GSO_TCPV6       4
+#define VIRTIO_NET_HDR_GSO_UDP         2
+#define VIRTIO_NET_HDR_GSO_TCPV6       3
+#define VIRTIO_NET_HDR_RSC_NONE        4
+#define VIRTIO_NET_HDR_RSC_TCPV4       5
+#define VIRTIO_NET_HDR_RSC_TCPV6       6
 #define VIRTIO_NET_HDR_GSO_ECN      0x80
         u8 gso_type;
         le16 hdr_len;
@@ -3256,6 +3264,9 @@
         le16 csum_start;
         le16 csum_offset;
         le16 num_buffers;
+        le16 coalesced_pkts;
+        le16 coalesced_size;
+        le16 dup_acks;
 };
 \end{lstlisting}

@@ -3272,6 +3283,12 @@
 when VIRTIO_NET_F_MRG_RXBUF was negotiated; without that feature the
 structure was 2 bytes shorter.

+The legacy driver only presented \field{coalesced_pkts}, 
\field{coalesced_size}
+and \field{dup_ack} in the struct virtio_net_hdr when 
VIRTIO_NET_F_GUEST_RSC4
+or VIRTIO_NET_F_GUEST_RSC6 was negotiated and a packet really has coalesced
+additional packet(s), without satisfying the 2 conditions the structure was
+6 bytes shorter.
+
 When using the legacy interface, the driver SHOULD ignore the
 \field{len} value in used ring entries for the transmit queues
 and the controlq queue.
@@ -3451,6 +3468,19 @@
 1514 bytes.  The 12-byte struct virtio_net_hdr is prepended to this,
 making for 65562 or 1526 bytes.

+If the VIRTIO_NET_F_GUEST_RSC4 feature is used, the maximum incoming
+packet will be up to 65550 bytes long (the maximum size of a TCP packet,
+plus the 14 byte ethernet header), otherwise 1514 bytes.
+The 16-byte struct virtio_net_hdr is prepended to this,
+making for 65566 or 1528 bytes.
+
+If the VIRTIO_NET_F_GUEST_RSC6 feature is used, the maximum incoming
+packet will be up to 65590 bytes long (the maximum size of a TCP packet,
+plus the 40-byte standard ipv6 header, plus the 14 byte ethernet header),
+otherwise 1514 bytes.
+The 16-byte struct virtio_net_hdr is prepended to this,
+making for 65606 or 1528 bytes.
+
 \drivernormative{\paragraph}{Setting Up Receive Buffers}{Device Types 
/ Network Device / Device Operation / Setting Up Receive Buffers}

 \begin{itemize}
@@ -3514,6 +3544,24 @@
   set: if so, device has validated the packet checksum.
   In case of multiple encapsulated protocols, one level of checksums
   has been validated.
+
+\item If one or both VIRTIO_NET_F_GUEST_RSC4, RSC6 were negotiated,
+  but it is a original packet which didn't coalesce any packet,
+  then the \field{flags} should be set to VIRTIO_NET_HDR_RSC_NONE.
+
+\item If the VIRTIO_NET_F_GUEST_RSC4, RSC6 options were negotiated,
+  and \field{flags} notifies it's a coalesced packet, then
+  \field{coalesced_pkts} indicates how many packets are coalesced into
+  this packet.
+
+\item If the VIRTIO_NET_F_GUEST_RSC4, RSC6 options were negotiated,
+  and \field{flags} notifies it's a coalesced packet, then
+  \field{coalesced_size} indicates the real packet size after coalesced.
+
+\item If the VIRTIO_NET_F_GUEST_RSC4, RSC6 options were negotiated,
+  and \field{flags} notifies it's a coalesced packet, then
+  \field{dup_ack} indicates how many duplicated ack packets are
+  coalesced into this packet.
 \end{enumerate}

 Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP and ECN
@@ -3626,6 +3674,15 @@
 If neither VIRTIO_NET_HDR_F_NEEDS_CSUM nor
 VIRTIO_NET_HDR_F_DATA_VALID is set, the driver MUST NOT
 rely on the packet checksum being correct.
+
+If one of VIRTIO_NET_F_GUEST_RSC4, RSC6 options have
+been negotiated, the driver MAY use \field{hdr_len} as a
+hint about the real virtio header size, for example, if a
+packets is an original packet, then the virtio header size
+should not contain the \field{coalesced_pkts}, \field{coalesced_size}
+and \field {dup_ack}, otherwise, these 3 fields should be included in
+virito header size.
+
 \subsubsection{Control Virtqueue}\label{sec:Device Types / Network 
Device / Device Operation / Control Virtqueue}

 The driver uses the control virtqueue (if VIRTIO_NET_F_CTRL_VQ is