block: add BlockLimits.max_iov field

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>

2 files changed, 10 insertions, 0 deletions

diff --git a/block/io.c b/block/io.c
index 841f5b503f..42050a0598 100644
--- a/block/io.c
+++ b/block/io.c
@@ -166,9 +166,13 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
         bs->bl.max_transfer_length = bs->file->bs->bl.max_transfer_length;
         bs->bl.min_mem_alignment = bs->file->bs->bl.min_mem_alignment;
         bs->bl.opt_mem_alignment = bs->file->bs->bl.opt_mem_alignment;
+        bs->bl.max_iov = bs->file->bs->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) {
@@ -189,6 +193,9 @@ void bdrv_refresh_limits(BlockDriverState *bs, Error **errp)
         bs->bl.min_mem_alignment =
             MAX(bs->bl.min_mem_alignment,
                 bs->backing->bs->bl.min_mem_alignment);
+        bs->bl.max_iov =
+            MIN(bs->bl.max_iov,
+                bs->backing->bs->bl.max_iov);
     }
 
     /* Then let the driver override it */
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 9a1c466c84..256609dd3d 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -330,6 +330,9 @@ 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;