[Qemu-devel] [PATCH v2 2/4] spec/vhost-user: extend vhost-user to support the vhost-pci based inter-vm communiaction

[Wei Wang]

The protocol feature, VHOST_USER_PROTOCOL_F_VHOST_PCI, indicates the
support of vhost-pci. With the vhost-pci extension, the slave may
actively sending meesages to the master. VHOST_USER_SET_FEATURES is one
of the examples.

To understand this example, let's first understand how
the device feature bits are negotiated between the slave
device/driver and the master device/driver:
1) the master device (e.g. virtio-net) GET_FEATURES from the slave
(assume the feature bits are "f1");
2) the master device negotiates the feature bits with its driver (assume
the device gets "f2" after the negotiation);
3) the master device SET_FEATURES("f2") to the slave;
4) the slave creates a slave device (e.g. vhost-pci-net) with "f2" and
the slave device negotiates "f2" with its driver (assume the device gets
"f3" after the negotiation);
5) the slave _actively_ SET_FEATURES("f3") to the master device;
6) if "f3" != "f2", the master device needs to perform a device reset
and re-negotiate the feature bits with its driver using "f3".

Signed-off-by: Wei Wang <address@hidden>
---
 docs/specs/vhost-user.txt | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt
index d70bd83..3bbe641 100644
--- a/docs/specs/vhost-user.txt
+++ b/docs/specs/vhost-user.txt
@@ -17,12 +17,15 @@ The protocol defines 2 sides of the communication, master 
and slave. Master is
 the application that shares its virtqueues, in our case QEMU. Slave is the
 consumer of the virtqueues.
 
-In the current implementation QEMU is the Master, and the Slave is intended to
+In the traditional implementation QEMU is the master, and the slave is 
intended to
 be a software Ethernet switch running in user space, such as Snabbswitch.
 
 Master and slave can be either a client (i.e. connecting) or server (listening)
 in the socket communication.
 
+The current vhost-user protocol is extended to support the vhost-pci based 
inter-VM
+communication. In this case, both the slave and master are QEMU instances.
+
 Message Specification
 ---------------------
 
@@ -36,7 +39,7 @@ consists of 3 header fields and a payload:
  * Request: 32-bit type of the request
  * Flags: 32-bit bit field:
    - Lower 2 bits are the version (currently 0x01)
-   - Bit 2 is the reply flag - needs to be sent on each reply from the slave
+   - Bit 2 is the reply flag - needs to be sent on each reply
    - Bit 3 is the need_reply flag - see VHOST_USER_PROTOCOL_F_REPLY_ACK for
      details.
  * Size - 32-bit size of the payload
@@ -119,9 +122,9 @@ The protocol for vhost-user is based on the existing 
implementation of vhost
 for the Linux Kernel. Most messages that can be sent via the Unix domain socket
 implementing vhost-user have an equivalent ioctl to the kernel implementation.
 
-The communication consists of master sending message requests and slave sending
-message replies. Most of the requests don't require replies. Here is a list of
-the ones that do:
+Traditionally, the communication consists of master sending message requests 
and
+slave sending message replies. Most of the requests don't require replies. Here
+is a list of the ones that do:
 
  * VHOST_USER_GET_FEATURES
  * VHOST_USER_GET_PROTOCOL_FEATURES
@@ -130,6 +133,10 @@ the ones that do:
 
 [ Also see the section on REPLY_ACK protocol extension. ]
 
+Currently, the communication also supports the slave actively sending messages
+to the master. Here is a list of them:
+ * VHOST_USER_SET_FEATURES
+
 There are several messages that the master sends with file descriptors passed
 in the ancillary data:
 
@@ -259,6 +266,7 @@ Protocol features
 #define VHOST_USER_PROTOCOL_F_LOG_SHMFD      1
 #define VHOST_USER_PROTOCOL_F_RARP           2
 #define VHOST_USER_PROTOCOL_F_REPLY_ACK      3
+#define VHOST_USER_PROTOCOL_F_VHOST_PCI      4
 
 Message types
 -------------
@@ -279,6 +287,7 @@ Message types
       Id: 2
       Ioctl: VHOST_SET_FEATURES
       Master payload: u64
+      Slave payload: u64
 
       Enable features in the underlying vhost implementation using a bitmask.
       Feature bit VHOST_USER_F_PROTOCOL_FEATURES signals slave support for
-- 
2.7.4