[Qemu-devel] [PATCH 3/5] raw_bsd: Don't advertise flags not supported by protocol layer

[Eric Blake <eblake@redhat.com>]

The raw format layer supports all flags via passthrough - but
it only makes sense to pass through flags that the lower layer
actually supports.

Thanks to the previous patch, the raw format layer now attempts
to fragment writes at the max_transfer limit it inherits from
the NBD protocol layer, recently set to 32m.  An attempt to do
'w -f 0 40m' to an NBD server that lacks FUA thus changed from
flushing once (after NBD fragmented a single 40m write itself)
to instead flushing twice (the format layer sees BDRV_REQ_FUA
in supported_write_flags, so it sends the flag on to both
fragments, and then the block layer emulates FUA by flushing
for both the 32m and 8m fragments at the protocol layer).
This patch fixes the performance regression (now that the
format layer no longer advertises a flag not present at the
protocol layer, the flush to emulate FUA is deferred to the
last fragment).

Note that 'w -f -z 0 40m' does not currently exhibit the same
problem, because there, the fragmentation does not occur until
at the NBD layer (the raw layer has .bdrv_co_pwrite_zeroes, and
the NBD layer doesn't advertise max_pwrite_zeroes to constrain
things at the raw layer) - but that problem is latent and would
have the same problem with too many flushes without this patch
once the NBD layer implements support for using the new
NBD_CMD_WRITE_ZEROES and sets max_pwrite_zeroes to the same 32m
limit as recommended by the NBD protocol.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/raw_bsd.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/block/raw_bsd.c b/block/raw_bsd.c
index 351ed2a..47a8352 100644
--- a/block/raw_bsd.c
+++ b/block/raw_bsd.c
@@ -197,8 +197,10 @@ static int raw_open(BlockDriverState *bs, QDict *options, int flags,
                     Error **errp)
 {
     bs->sg = bs->file->bs->sg;
-    bs->supported_write_flags = BDRV_REQ_FUA;
-    bs->supported_zero_flags = BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP;
+    bs->supported_write_flags = BDRV_REQ_FUA &
+        bs->file->bs->supported_write_flags;
+    bs->supported_zero_flags = (BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP) &
+        bs->file->bs->supported_zero_flags;

     if (bs->probed && !bdrv_is_read_only(bs)) {
         fprintf(stderr,
-- 
2.5.5