[RFC PATCH v5 ] virtio-spec: Introduce new Receive-Segment-Coalescing feature for guest

[Wei Xu <wexu@redhat.com>]

Hi There,
This is the updated specification for Receive Segment Coalescing feature.

Changelog:
v4->v5
-- Redefine support of the feature bit from
   by 'VIRTIO_NET_F_CTRL_GUEST_OFFLOADS' feature bit to
   VIRTIO_NET_GUEST_RSC4/6.
-- Add varying of the header fields description in case of
   if or not the feature is negotiated.
-- Add supporting of dynamically control of the feature by
   'VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET' control command.


v3->v4
-- Remove maximum packets size section.
-- Remove useless 'rsc_size' field after checked out the Microsoft whql 
spec again.


v2->v3:
-- Revise flag definition for RSC4/6 to start from 5, add a 'RESERVED'
definition for already used number '2' which stands for historical
meaning of 'VIRTIO_NET_HDR_GSO_TCPV4_ECN'.
-- Add prefix 'rsc_' to new fields in virtio_net_hdr.
-- Drop wrong legacy part.
-- Revise maximum payload size for coalesced packets, also revise max
ip payload from 65536 to 65535 for TSO4/6/UFO.
-- Add default value of new fields member as zero for original packets.
-- Drop redundant hint in 'hdr_len' for RSC header size, both
send/receive should use the same header after the feature was
acknowledged.

v1->v2:
Feature 22 should not be used by device, replacing it with 41 and 42 for
VIRTIO_NET_GUEST_RSC4 and VIRTIO_NET_GUEST_RSC6.



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 (41)] Driver can receive coalesced IPv4 
tcp packets.
+
+\item[VIRTIO_NET_F_GUEST_RSC6 (42)] 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 +3252,12 @@
         u8 flags;
 #define VIRTIO_NET_HDR_GSO_NONE        0
 #define VIRTIO_NET_HDR_GSO_TCPV4       1
+#define VIRTIO_NET_HDR_RESERVED        2
 #define VIRTIO_NET_HDR_GSO_UDP         3
 #define VIRTIO_NET_HDR_GSO_TCPV6       4
+#define VIRTIO_NET_HDR_RSC_NONE        5
+#define VIRTIO_NET_HDR_RSC_TCPV4       6
+#define VIRTIO_NET_HDR_RSC_TCPV6       7
 #define VIRTIO_NET_HDR_GSO_ECN      0x80
         u8 gso_type;
         le16 hdr_len;
@@ -3256,6 +3265,8 @@
         le16 csum_start;
         le16 csum_offset;
         le16 num_buffers;
+        le16 rsc_pkts;
+        le16 rsc_dup_acks;
 };
 \end{lstlisting}

@@ -3514,6 +3525,24 @@
   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 have been negotiated,
+  while an original packet which have not coalesced any packet is going to
+  be sent, then the \field{flags} should be set to VIRTIO_NET_HDR_RSC_NONE.
+
+\item If one or both VIRTIO_NET_F_GUEST_RSC4, RSC6 features have been 
negotiated,
+  and \field{flags} is either RSC4 or RSC6, then \field{rsc_pkts} indicates
+  how many packets have been coalesced into this packet. Otherwise it 
should
+  be zero.
+
+\item If one or both VIRTIO_NET_F_GUEST_RSC4, RSC6 features have been 
negotiated,
+  and \field{flags} is either RSC4 or RSC6, then \field{rsc_dup_acks} 
indicates
+  how many duplicated ack packets have been coalesced into this packet. 
Otherwise
+  it should be zero.
+
+\item If none of VIRTIO_NET_F_GUEST_RSC4, RSC6 feature bits have been 
negotiated,
+  the 'rsc_pkts' and 'rsc_dup_acks' fields should not be presented in the
+  'virtio_net_hdr', which means the header is 4 bytes shorter.
 \end{enumerate}

 Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP and ECN
@@ -3959,6 +3988,8 @@
 #define VIRTIO_NET_F_GUEST_TSO6       8
 #define VIRTIO_NET_F_GUEST_ECN        9
 #define VIRTIO_NET_F_GUEST_UFO        10
+#define VIRTIO_NET_F_GUEST_RSC4       41
+#define VIRTIO_NET_F_GUEST_RSC6       42

 #define VIRTIO_NET_CTRL_GUEST_OFFLOADS       5
  #define VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET   0