[Qemu-devel] [PATCH 1/1] block: align bounce buffers to page

["Denis V. Lunev" <den@openvz.org>]

The following sequence
    int fd = open(argv[1], O_RDWR | O_CREAT | O_DIRECT, 0644);
    for (i = 0; i < 100000; i++)
            write(fd, buf, 4096);
performs 5% better if buf is aligned to 4096 bytes.

The difference is quite reliable.

On the other hand we do not want at the moment to enforce bounce
buffering if guest request is aligned to 512 bytes. This patch
forces page alignment when we are really forced to perform bounce
buffering.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
---
 block.c                   | 19 +++++++++++++++++--
 block/raw-posix.c         |  3 ++-
 include/block/block.h     |  2 ++
 include/block/block_int.h |  3 +++
 4 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index d45e4dd..d17c5ef 100644
--- a/block.c
+++ b/block.c
@@ -232,6 +232,16 @@ size_t bdrv_opt_mem_align(BlockDriverState *bs)
     return bs->bl.opt_mem_alignment;
 }
 
+size_t bdrv_min_mem_align(BlockDriverState *bs)
+{
+    if (!bs || !bs->drv) {
+        /* 4k should be on the safe side */
+        return 4096;
+    }
+
+    return bs->bl.min_mem_alignment;
+}
+
 /* check if the path starts with "<protocol>:" */
 int path_has_protocol(const char *path)
 {
@@ -542,8 +552,10 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
         bs->bl.opt_transfer_length = bs->file->bl.opt_transfer_length;
         bs->bl.max_transfer_length = bs->file->bl.max_transfer_length;
         bs->bl.opt_mem_alignment = bs->file->bl.opt_mem_alignment;
+        bs->bl.min_mem_alignment = bs->file->bl.min_mem_alignment;
     } else {
-        bs->bl.opt_mem_alignment = 512;
+        bs->bl.opt_mem_alignment = 4096;
+        bs->bl.min_mem_alignment = 512;
     }
 
     if (bs->backing_hd) {
@@ -561,6 +573,9 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
         bs->bl.opt_mem_alignment =
             MAX(bs->bl.opt_mem_alignment,
                 bs->backing_hd->bl.opt_mem_alignment);
+        bs->bl.min_mem_alignment =
+            MAX(bs->bl.min_mem_alignment,
+                bs->backing_hd->bl.min_mem_alignment);
     }
 
     /* Then let the driver override it */
@@ -5331,7 +5346,7 @@ void *qemu_try_blockalign0(BlockDriverState *bs, size_t size)
 bool bdrv_qiov_is_aligned(BlockDriverState *bs, QEMUIOVector *qiov)
 {
     int i;
-    size_t alignment = bdrv_opt_mem_align(bs);
+    size_t alignment = bdrv_min_mem_align(bs);
 
     for (i = 0; i < qiov->niov; i++) {
         if ((uintptr_t) qiov->iov[i].iov_base % alignment) {
diff --git a/block/raw-posix.c b/block/raw-posix.c
index 933c778..3ad2e5e 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -667,7 +667,8 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
     BDRVRawState *s = bs->opaque;
 
     raw_probe_alignment(bs, s->fd, errp);
-    bs->bl.opt_mem_alignment = s->buf_align;
+    bs->bl.min_mem_alignment = s->buf_align;
+    bs->bl.opt_mem_alignment = MAX(sysconf(_SC_PAGESIZE), s->buf_align);
 
     raw_probe_max_write_zeroes(bs);
 }
diff --git a/include/block/block.h b/include/block/block.h
index 3082d2b..b9b24b5 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -424,6 +424,8 @@ void bdrv_img_create(const char *filename, const char *fmt,
 
 /* Returns the alignment in bytes that is required so that no bounce buffer
  * is required throughout the stack */
+size_t bdrv_min_mem_align(BlockDriverState *bs);
+/* Returns optimal alignment in bytes for bounce buffer */
 size_t bdrv_opt_mem_align(BlockDriverState *bs);
 void bdrv_set_guest_block_size(BlockDriverState *bs, int align);
 void *qemu_blockalign(BlockDriverState *bs, size_t size);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index e264be9..7dccac4 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -296,6 +296,9 @@ typedef struct BlockLimits {
     int max_transfer_length;
 
     /* memory alignment so that no bounce buffer is needed */
+    size_t min_mem_alignment;
+
+    /* memory alignment for effective transfers when bounce buffer is needed */
     size_t opt_mem_alignment;
 } BlockLimits;
 
-- 
1.9.1