[Qemu-devel] [PATCH v3 1/2] vhost-user: Introduce a new protocol feature REPLY_ACK.

[Prerna Saxena <saxenap.ltc@gmail.com>]

From: Prerna Saxena <prerna.saxena@nutanix.com>

This introduces the VHOST_USER_PROTOCOL_F_REPLY_ACK.

If negotiated, client applications should send a u64 payload in
response to any message that contains the "need_response" bit set
on the message flags. Setting the payload to "zero" indicates the
command finished successfully. Likewise, setting it to "non-zero"
indicates an error.

Currently implemented only for SET_MEM_TABLE.

Signed-off-by: Prerna Saxena <prerna.saxena@nutanix.com>
---
 docs/specs/vhost-user.txt | 44 ++++++++++++++++++++++++++++++++++++++++++++
 hw/virtio/vhost-user.c    | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+)

diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt
index 777c49c..26dbe71 100644
--- a/docs/specs/vhost-user.txt
+++ b/docs/specs/vhost-user.txt
@@ -37,6 +37,8 @@ consists of 3 header fields and a payload:
  * 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 3 is the need_response flag - see VHOST_USER_PROTOCOL_F_REPLY_ACK for
+     details.
  * Size - 32-bit size of the payload
 
 
@@ -126,6 +128,8 @@ the ones that do:
  * VHOST_GET_VRING_BASE
  * VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD)
 
+[ Also see the section on REPLY_ACK protocol extension. ]
+
 There are several messages that the master sends with file descriptors passed
 in the ancillary data:
 
@@ -254,6 +258,7 @@ Protocol features
 #define VHOST_USER_PROTOCOL_F_MQ             0
 #define VHOST_USER_PROTOCOL_F_LOG_SHMFD      1
 #define VHOST_USER_PROTOCOL_F_RARP           2
+#define VHOST_USER_PROTOCOL_F_REPLY_ACK      3
 
 Message types
 -------------
@@ -464,3 +469,42 @@ Message types
       is present in VHOST_USER_GET_PROTOCOL_FEATURES.
       The first 6 bytes of the payload contain the mac address of the guest to
       allow the vhost user backend to construct and broadcast the fake RARP.
+
+VHOST_USER_PROTOCOL_F_REPLY_ACK:
+-------------------------------
+The original vhost-user specification only demands responses for certain
+commands. This differs from the vhost protocol implementation where commands
+are sent over an ioctl() call and block until the client has completed.
+
+With this protocol extension negotiated, the sender (QEMU) can set the newly
+introduced "need_response" [Bit 3] flag to any command. This indicates that
+the client MUST respond with a Payload VhostUserMsg indicating success or
+failure. The payload should be set to zero on success or non-zero on failure.
+In other words, response must be in the following format :
+
+------------------------------------
+| request | flags | size | payload |
+------------------------------------
+
+ * Request: 32-bit type of the request
+ * Flags: 32-bit bit field:
+ * Size: size of the payload ( see below)
+ * Payload : a u64 integer, where a non-zero value indicates a failure.
+
+This aids debugging the application's responses from QEMU. More
+importantly, it indicates to QEMU that the requested operation has
+deterministically (not) been met. Today, QEMU is expected to terminate
+the main vhost-user loop upon receiving such errors. In future, qemu could
+be taught to be more resilient for selective requests.
+
+Note that as per the original vhost-user protocol, the following four messages
+anyway require distinct responses from the vhost-user client process:
+ * VHOST_GET_FEATURES
+ * VHOST_GET_PROTOCOL_FEATURES
+ * VHOST_GET_VRING_BASE
+ * VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD)
+
+For these message types, the presence of VHOST_USER_PROTOCOL_F_REPLY_ACK or
+need_response bit being set brings no behaviourial change.
+The response from the client is identical whether or not the REPLY_ACK feature
+has been negotiated.
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 495e09f..899f354 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -31,6 +31,7 @@ enum VhostUserProtocolFeature {
     VHOST_USER_PROTOCOL_F_MQ = 0,
     VHOST_USER_PROTOCOL_F_LOG_SHMFD = 1,
     VHOST_USER_PROTOCOL_F_RARP = 2,
+    VHOST_USER_PROTOCOL_F_REPLY_ACK = 3,
 
     VHOST_USER_PROTOCOL_F_MAX
 };
@@ -84,6 +85,7 @@ typedef struct VhostUserMsg {
 
 #define VHOST_USER_VERSION_MASK     (0x3)
 #define VHOST_USER_REPLY_MASK       (0x1<<2)
+#define VHOST_USER_NEED_RESPONSE_MASK       (0x1 << 3)
     uint32_t flags;
     uint32_t size; /* the following payload size */
     union {
@@ -107,6 +109,25 @@ static VhostUserMsg m __attribute__ ((unused));
 /* The version of the protocol we support */
 #define VHOST_USER_VERSION    (0x1)
 
+static int process_message_reply(struct vhost_dev *dev,
+                                    VhostUserRequest request)
+{
+    VhostUserMsg msg;
+
+    if (vhost_user_read(dev, &msg) < 0) {
+        return 0;
+    }
+
+    if (msg.request != request) {
+        error_report("Received unexpected msg type."
+                        "Expected %d received %d",
+                        request, msg.request);
+        return -1;
+    }
+
+    return msg.payload.u64 ? -1 : 0;
+}
+
 static bool ioeventfd_enabled(void)
 {
     return kvm_enabled() && kvm_eventfds_enabled();
@@ -239,11 +260,18 @@ static int vhost_user_set_mem_table(struct vhost_dev *dev,
     int fds[VHOST_MEMORY_MAX_NREGIONS];
     int i, fd;
     size_t fd_num = 0;
+    bool reply_supported = virtio_has_feature(dev->protocol_features,
+                            VHOST_USER_PROTOCOL_F_REPLY_ACK);
+
     VhostUserMsg msg = {
         .request = VHOST_USER_SET_MEM_TABLE,
         .flags = VHOST_USER_VERSION,
     };
 
+    if (reply_supported) {
+        msg.flags |= VHOST_USER_NEED_RESPONSE_MASK;
+    }
+
     for (i = 0; i < dev->mem->nregions; ++i) {
         struct vhost_memory_region *reg = dev->mem->regions + i;
         ram_addr_t offset;
@@ -277,6 +305,10 @@ static int vhost_user_set_mem_table(struct vhost_dev *dev,
 
     vhost_user_write(dev, &msg, fds, fd_num);
 
+    if (reply_supported) {
+        return process_message_reply(dev, msg.request);
+    }
+
     return 0;
 }
 
-- 
1.8.1.2