[Qemu-devel] [PATCH v3 02/22] block: Document supported flags during bdrv_aligned_preadv()

[Eric Blake <eblake@redhat.com>]

We don't pass any flags on to drivers to handle.  Tighten an
assert to explain why we pass 0 to bdrv_driver_preadv(), and add
some comments on things to be aware of if we want to turn on
per-BDS BDRV_REQ_FUA support during reads in the future.  Also,
document that we may want to consider using unmap during
copy-on-read operations where the read is all zeroes.

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>

---
v3: no change
v2: retitle from 'Honor flags during bdrv_aligned_preadv()', and
change scope of patch
---
 block/io.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/block/io.c b/block/io.c
index b95e856..994d3fa 100644
--- a/block/io.c
+++ b/block/io.c
@@ -945,6 +945,9 @@ static int coroutine_fn bdrv_co_do_copy_on_readv(BlockDriverState *bs,

     if (drv->bdrv_co_pwrite_zeroes &&
         buffer_is_zero(bounce_buffer, iov.iov_len)) {
+        /* FIXME: Should we (perhaps conditionally) be setting
+         * BDRV_REQ_MAY_UNMAP, if it will allow for a sparser copy
+         * that still correctly reads as zero? */
         ret = bdrv_co_do_pwrite_zeroes(bs, cluster_offset, cluster_bytes, 0);
     } else {
         /* This does not change the data on the disk, it is not necessary
@@ -987,7 +990,12 @@ static int coroutine_fn bdrv_aligned_preadv(BlockDriverState *bs,
     assert((bytes & (align - 1)) == 0);
     assert(!qiov || bytes == qiov->size);
     assert((bs->open_flags & BDRV_O_NO_IO) == 0);
-    assert(!(flags & ~BDRV_REQ_MASK));
+
+    /* TODO: We would need a per-BDS .supported_read_flags and
+     * potential fallback support, if we ever implement any read flags
+     * to pass through to drivers.  For now, there aren't any
+     * passthrough flags.  */
+    assert(!(flags & ~(BDRV_REQ_NO_SERIALISING | BDRV_REQ_COPY_ON_READ)));

     /* Handle Copy on Read and associated serialisation */
     if (flags & BDRV_REQ_COPY_ON_READ) {
-- 
2.5.5