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

[Stefan Hajnoczi]

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

Instead of assuming raw-posix and hardcoding IOV_MAX in several places,
put the limit into BlockLimits.

Cc: Peter Lieven <address@hidden>
Suggested-by: Kevin Wolf <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
Peter Lieven: I think the SCSI LUN level does not have a maximum
scatter-gather segments constraint.  That is probably only at the HBA
level.  CCed you anyway in case you think block/iscsi.c should set the
max_iov field.

Kevin: The default is now INT_MAX.  This means non-raw-posix users will
now be able to merge more requests than before.  They were limited to
IOV_MAX previously.  This could expose limits in other BlockDrivers
which we weren't aware of...
---
 block/io.c                | 3 +++
 block/raw-posix.c         | 1 +
 include/block/block_int.h | 3 +++
 3 files changed, 7 insertions(+)

diff --git a/block/io.c b/block/io.c
index e295992..6750de6 100644
--- a/block/io.c
+++ b/block/io.c
@@ -165,9 +165,11 @@ 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();
+        bs->bl.max_iov = INT_MAX;
     }
 
     if (bs->backing_hd) {
@@ -188,6 +190,7 @@ 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/block/raw-posix.c b/block/raw-posix.c
index cbe6574..faa6ae0 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -735,6 +735,7 @@ static void raw_refresh_limits(BlockDriverState *bs, Error 
**errp)
     raw_probe_alignment(bs, s->fd, errp);
     bs->bl.min_mem_alignment = s->buf_align;
     bs->bl.opt_mem_alignment = MAX(s->buf_align, getpagesize());
+    bs->bl.max_iov = IOV_MAX; /* limit comes from preadv()/pwritev()/etc */
 }
 
 static int check_for_dasd(int fd)
diff --git a/include/block/block_int.h b/include/block/block_int.h
index b0476fc..767e83d 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