nbd/server: introduce NBD_CMD_CACHE

Handle nbd CACHE command. Just do read, without sending read data back.
Cache mechanism should be done by exported node driver chain.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180413143156.11409-1-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: fix two missing case labels in switch statements]
Signed-off-by: Eric Blake <eblake@redhat.com>

include/block/nbd.h

@@ -135,6 +135,7 @@ typedef struct NBDExtent {
 #define NBD_FLAG_SEND_TRIM         (1 << 5) /* Send TRIM (discard) */
 #define NBD_FLAG_SEND_WRITE_ZEROES (1 << 6) /* Send WRITE_ZEROES */
 #define NBD_FLAG_SEND_DF           (1 << 7) /* Send DF (Do not Fragment) */
+#define NBD_FLAG_SEND_CACHE        (1 << 8) /* Send CACHE (prefetch) */
 
 /* New-style handshake (global) flags, sent from server to client, and
    control what will happen during handshake phase. */
@@ -195,7 +196,7 @@ enum {
     NBD_CMD_DISC = 2,
     NBD_CMD_FLUSH = 3,
     NBD_CMD_TRIM = 4,
-    /* 5 reserved for failed experiment NBD_CMD_CACHE */
+    NBD_CMD_CACHE = 5,
     NBD_CMD_WRITE_ZEROES = 6,
     NBD_CMD_BLOCK_STATUS = 7,
 };

nbd/common.c

@@ -148,6 +148,8 @@ const char *nbd_cmd_lookup(uint16_t cmd)
         return "flush";
     case NBD_CMD_TRIM:
         return "trim";
+    case NBD_CMD_CACHE:
+        return "cache";
     case NBD_CMD_WRITE_ZEROES:
         return "write zeroes";
     case NBD_CMD_BLOCK_STATUS:

nbd/server.c

@@ -1252,7 +1252,7 @@ static coroutine_fn int nbd_negotiate(NBDClient *client, Error **errp)
     int ret;
     const uint16_t myflags = (NBD_FLAG_HAS_FLAGS | NBD_FLAG_SEND_TRIM |
                               NBD_FLAG_SEND_FLUSH | NBD_FLAG_SEND_FUA |
-                              NBD_FLAG_SEND_WRITE_ZEROES);
+                              NBD_FLAG_SEND_WRITE_ZEROES | NBD_FLAG_SEND_CACHE);
     bool oldStyle;
 
     /* Old style negotiation header, no room for options
@@ -2034,7 +2034,9 @@ static int nbd_co_receive_request(NBDRequestData *req, NBDRequest *request,
         return -EIO;
     }
 
-    if (request->type == NBD_CMD_READ || request->type == NBD_CMD_WRITE) {
+    if (request->type == NBD_CMD_READ || request->type == NBD_CMD_WRITE ||
+        request->type == NBD_CMD_CACHE)
+    {
         if (request->len > NBD_MAX_BUFFER_SIZE) {
             error_setg(errp, "len (%" PRIu32" ) is larger than max len (%u)",
                        request->len, NBD_MAX_BUFFER_SIZE);
@@ -2119,7 +2121,7 @@ static coroutine_fn int nbd_do_cmd_read(NBDClient *client, NBDRequest *request,
     int ret;
     NBDExport *exp = client->exp;
 
-    assert(request->type == NBD_CMD_READ);
+    assert(request->type == NBD_CMD_READ || request->type == NBD_CMD_CACHE);
 
     /* XXX: NBD Protocol only documents use of FUA with WRITE */
     if (request->flags & NBD_CMD_FLAG_FUA) {
@@ -2138,7 +2140,7 @@ static coroutine_fn int nbd_do_cmd_read(NBDClient *client, NBDRequest *request,
 
     ret = blk_pread(exp->blk, request->from + exp->dev_offset, data,
                     request->len);
-    if (ret < 0) {
+    if (ret < 0 || request->type == NBD_CMD_CACHE) {
         return nbd_send_generic_reply(client, request->handle, ret,
                                       "reading from file failed", errp);
     }
@@ -2171,6 +2173,7 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
 
     switch (request->type) {
     case NBD_CMD_READ:
+    case NBD_CMD_CACHE:
         return nbd_do_cmd_read(client, request, data, errp);
 
     case NBD_CMD_WRITE: