block: Make supported_write_flags a per-bds property

Pre-patch, .supported_write_flags lives at the driver level, which
means we are blindly declaring that all block devices using a
given driver will either equally support FUA, or that we need a
fallback at the block layer.  But there are drivers where FUA
support is a per-block decision: the NBD block driver is dependent
on the remote server advertising NBD_FLAG_SEND_FUA (and has
fallback code to duplicate the flush that the block layer would do
if NBD had not set .supported_write_flags); and the iscsi block
driver is dependent on the mode sense bits advertised by the
underlying device (and is currently silently ignoring FUA requests
if the underlying device does not support FUA).

The fix is to make supported flags as a per-BDS option, set during
.bdrv_open().  This patch moves the variable and fixes NBD and iscsi
to set it only conditionally; later patches will then further
simplify the NBD driver to quit duplicating work done at the block
layer, as well as tackle the fact that SCSI does not support FUA
semantics on WRITESAME(10/16) but only on WRITE(10/16).

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

1 files changed, 2 insertions, 2 deletions

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 6fbe648231..10f4962fe8 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -158,8 +158,6 @@ struct BlockDriver {
     int coroutine_fn (*bdrv_co_pwritev)(BlockDriverState *bs,
         uint64_t offset, uint64_t bytes, QEMUIOVector *qiov, int flags);
 
-    int supported_write_flags;
-
     /*
      * Efficiently zero a region of the disk image.  Typically an image format
      * would use a compact metadata representation to implement this.  This
@@ -445,6 +443,8 @@ struct BlockDriverState {
 
     /* Alignment requirement for offset/length of I/O requests */
     unsigned int request_alignment;
+    /* Flags honored during pwrite (so far: BDRV_REQ_FUA) */
+    unsigned int supported_write_flags;
 
     /* the following member gives a name to every node on the bs graph. */
     char node_name[32];