iscsi: Advertise realistic limits to block layer

The function sector_limits_lun2qemu() returns a value in units of
the block layer's 512-byte sector, and can be as large as
0x40000000, which is much larger than the block layer's inherent
limit of BDRV_REQUEST_MAX_SECTORS.  The block layer already
handles '0' as a synonym to the inherent limit, and it is nicer
to return this value than it is to calculate an arbitrary
maximum, for two reasons: we want to ensure that the block layer
continues to special-case '0' as 'no limit beyond the inherent
limits'; and we want to be able to someday expand the block
layer to allow 64-bit limits, where auditing for uses of
BDRV_REQUEST_MAX_SECTORS will help us make sure we aren't
artificially constraining iscsi to old block layer limits.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/block/iscsi.c b/block/iscsi.c
index 9bb5ff6216..c5dedb3407 100644
--- a/block/iscsi.c
+++ b/block/iscsi.c
@@ -1698,7 +1698,9 @@ static void iscsi_close(BlockDriverState *bs)
 
 static int sector_limits_lun2qemu(int64_t sector, IscsiLun *iscsilun)
 {
-    return MIN(sector_lun2qemu(sector, iscsilun), INT_MAX / 2 + 1);
+    int limit = MIN(sector_lun2qemu(sector, iscsilun), INT_MAX / 2 + 1);
+
+    return limit < BDRV_REQUEST_MAX_SECTORS ? limit : 0;
 }
 
 static void iscsi_refresh_limits(BlockDriverState *bs, Error **errp)