[Qemu-devel] [PATCH 1/3] block: implement blk_co_pcache

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Do effective copy-on-read request when we don't need data actually. It
will be used for block-stream and NBD_CMD_CACHE.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block.h          |  8 +++++++-
 include/sysemu/block-backend.h |  7 +++++++
 block/io.c                     | 18 ++++++++++++------
 3 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index f9415ed740..4662f25513 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -88,8 +88,14 @@ typedef enum {
      * fallback. */
     BDRV_REQ_NO_FALLBACK        = 0x100,
 
+    /*
+     * BDRV_REQ_CACHE may be used only together with BDRV_REQ_COPY_ON_READ
+     * on read request and means that caller don't really need data to be
+     * written to qiov parameter which may be NULL.
+     */
+    BDRV_REQ_CACHE  = 0x200,
     /* Mask of valid flags */
-    BDRV_REQ_MASK               = 0x1ff,
+    BDRV_REQ_MASK               = 0x3ff,
 } BdrvRequestFlags;
 
 typedef struct BlockSizes {
diff --git a/include/sysemu/block-backend.h b/include/sysemu/block-backend.h
index 733c4957eb..2bcecd1190 100644
--- a/include/sysemu/block-backend.h
+++ b/include/sysemu/block-backend.h
@@ -135,6 +135,13 @@ static inline int coroutine_fn blk_co_pread(BlockBackend *blk, int64_t offset,
     return blk_co_preadv(blk, offset, bytes, &qiov, flags);
 }
 
+static inline int coroutine_fn blk_co_pcache(BlockBackend *blk, int64_t offset,
+                                             unsigned int bytes)
+{
+    return blk_co_preadv(blk, offset, bytes, NULL,
+                         BDRV_REQ_COPY_ON_READ | BDRV_REQ_CACHE);
+}
+
 static inline int coroutine_fn blk_co_pwrite(BlockBackend *blk, int64_t offset,
                                              unsigned int bytes, void *buf,
                                              BdrvRequestFlags flags)
diff --git a/block/io.c b/block/io.c
index 9ba1bada36..9daf7332bd 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1104,7 +1104,8 @@ bdrv_driver_pwritev_compressed(BlockDriverState *bs, uint64_t offset,
 }
 
 static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
-        int64_t offset, unsigned int bytes, QEMUIOVector *qiov)
+        int64_t offset, unsigned int bytes, QEMUIOVector *qiov,
+        int flags)
 {
     BlockDriverState *bs = child->bs;
 
@@ -1215,9 +1216,11 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BdrvChild *child,
                 goto err;
             }
 
-            qemu_iovec_from_buf(qiov, progress, bounce_buffer + skip_bytes,
-                                pnum - skip_bytes);
-        } else {
+            if (!(flags & BDRV_REQ_CACHE)) {
+                qemu_iovec_from_buf(qiov, progress, bounce_buffer + skip_bytes,
+                                    pnum - skip_bytes);
+            }
+        } else if (!(flags & BDRV_REQ_CACHE)) {
             /* Read directly into the destination */
             qemu_iovec_init(&local_qiov, qiov->niov);
             qemu_iovec_concat(&local_qiov, qiov, progress, pnum - skip_bytes);
@@ -1268,7 +1271,8 @@ static int coroutine_fn bdrv_aligned_preadv(BdrvChild *child,
      * potential fallback support, if we ever implement any read flags
      * to pass through to drivers.  For now, there aren't any
      * passthrough flags.  */
-    assert(!(flags & ~(BDRV_REQ_NO_SERIALISING | BDRV_REQ_COPY_ON_READ)));
+    assert(!(flags & ~(BDRV_REQ_NO_SERIALISING | BDRV_REQ_COPY_ON_READ |
+                       BDRV_REQ_CACHE)));
 
     /* Handle Copy on Read and associated serialisation */
     if (flags & BDRV_REQ_COPY_ON_READ) {
@@ -1296,7 +1300,9 @@ static int coroutine_fn bdrv_aligned_preadv(BdrvChild *child,
         }
 
         if (!ret || pnum != bytes) {
-            ret = bdrv_co_do_copy_on_readv(child, offset, bytes, qiov);
+            ret = bdrv_co_do_copy_on_readv(child, offset, bytes, qiov, flags);
+            goto out;
+        } else if (flags & BDRV_REQ_CACHE) {
             goto out;
         }
     }
-- 
2.18.0