block/io: Ignore FUA with cache.no-flush=on

For block drivers that don't advertise FUA support, we already call
bdrv_co_flush(), which considers BDRV_O_NO_FLUSH. However, drivers that
do support FUA still see the FUA flag with BDRV_O_NO_FLUSH and get the
associated performance penalty that cache.no-flush=on was supposed to
avoid.

Clear FUA for write requests if BDRV_O_NO_FLUSH is set.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250307221634.71951-3-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -1058,6 +1058,10 @@ bdrv_driver_pwritev(BlockDriverState *bs, int64_t offset, int64_t bytes,
         return -ENOMEDIUM;
     }
 
+    if (bs->open_flags & BDRV_O_NO_FLUSH) {
+        flags &= ~BDRV_REQ_FUA;
+    }
+
     if ((flags & BDRV_REQ_FUA) &&
         (~bs->supported_write_flags & BDRV_REQ_FUA)) {
         flags &= ~BDRV_REQ_FUA;