[Qemu-devel] [PATCH for-2.5 v2 1/4] block: add BlockLimits.max_iov field

[Stefan Hajnoczi <stefanha@redhat.com>]

The maximum number of struct iovec elements depends on the
BlockDriverState.  The raw-posix and iSCSI protocols have a maximum of
IOV_MAX but others could have different values.

Cc: Peter Lieven <pl@kamp.de>
Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
v2:
 * Change default from INT_MAX to IOV_MAX [Peter Lieven]
---
 block/io.c                | 5 +++++
 include/block/block_int.h | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/block/io.c b/block/io.c
index d4bc83b..37bc0dc 100644
--- a/block/io.c
+++ b/block/io.c
@@ -165,9 +165,13 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
         bs->bl.max_transfer_length = bs->file->bl.max_transfer_length;
         bs->bl.min_mem_alignment = bs->file->bl.min_mem_alignment;
         bs->bl.opt_mem_alignment = bs->file->bl.opt_mem_alignment;
+        bs->bl.max_iov = bs->file->bl.max_iov;
     } else {
         bs->bl.min_mem_alignment = 512;
         bs->bl.opt_mem_alignment = getpagesize();
+
+        /* Safe default since most protocols use readv()/writev()/etc */
+        bs->bl.max_iov = IOV_MAX;
     }
 
     if (bs->backing_hd) {
@@ -188,6 +192,7 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
         bs->bl.min_mem_alignment =
             MAX(bs->bl.min_mem_alignment,
                 bs->backing_hd->bl.min_mem_alignment);
+        bs->bl.max_iov = MIN(bs->bl.max_iov, bs->backing_hd->bl.max_iov);
     }
 
     /* Then let the driver override it */
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 8996baf..7d578c4 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -315,6 +315,9 @@ typedef struct BlockLimits {
 
     /* memory alignment for bounce buffer */
     size_t opt_mem_alignment;
+
+    /* maximum number of iovec elements */
+    int max_iov;
 } BlockLimits;
 
 typedef struct BdrvOpBlocker BdrvOpBlocker;
-- 
2.4.3