virtio-blk: introduce multiread

this patch finally introduces multiread support to virtio-blk. While
multiwrite support was there for a long time, read support was missing.

The complete merge logic is moved into virtio-blk.c which has
been the only user of request merging ever since. This is required
to be able to merge chunks of requests and immediately invoke callbacks
for those requests. Secondly, this is required to switch to
direct invocation of coroutines which is planned at a later stage.

The following benchmarks show the performance of running fio with
4 worker threads on a local ram disk. The numbers show the average
of 10 test runs after 1 run as warmup phase.

              |        4k        |       64k        |        4k
MB/s          | rd seq | rd rand | rd seq | rd rand | wr seq | wr rand
--------------+--------+---------+--------+---------+--------+--------
master        | 1221   | 1187    | 4178   | 4114    | 1745   | 1213
multiread     | 1829   | 1189    | 4639   | 4110    | 1894   | 1216

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 11 insertions, 8 deletions

diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
index 6ef3fa5..2027a64 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -134,29 +134,32 @@ typedef struct VirtIOBlock {
     struct VirtIOBlockDataPlane *dataplane;
 } VirtIOBlock;
 
-#define VIRTIO_BLK_MAX_MERGE_REQS 32
-
-typedef struct MultiReqBuffer {
-    BlockRequest        blkreq[VIRTIO_BLK_MAX_MERGE_REQS];
-    unsigned int        num_writes;
-} MultiReqBuffer;
-
 typedef struct VirtIOBlockReq {
+    int64_t sector_num;
     VirtIOBlock *dev;
     VirtQueueElement elem;
     struct virtio_blk_inhdr *in;
     struct virtio_blk_outhdr out;
     QEMUIOVector qiov;
     struct VirtIOBlockReq *next;
+    struct VirtIOBlockReq *mr_next;
     BlockAcctCookie acct;
 } VirtIOBlockReq;
 
+#define VIRTIO_BLK_MAX_MERGE_REQS 32
+
+typedef struct MultiReqBuffer {
+    VirtIOBlockReq *reqs[VIRTIO_BLK_MAX_MERGE_REQS];
+    unsigned int num_reqs;
+    bool is_write;
+} MultiReqBuffer;
+
 VirtIOBlockReq *virtio_blk_alloc_request(VirtIOBlock *s);
 
 void virtio_blk_free_request(VirtIOBlockReq *req);
 
 void virtio_blk_handle_request(VirtIOBlockReq *req, MultiReqBuffer *mrb);
 
-void virtio_submit_multiwrite(BlockBackend *blk, MultiReqBuffer *mrb);
+void virtio_blk_submit_multireq(BlockBackend *blk, MultiReqBuffer *mrb);
 
 #endif