[RFC PATCH v3 27/30] monitor: fdset: Match against O_DIRECT

[Fabiano Rosas]

We're about to enable the use of O_DIRECT in the migration code and
due to the alignment restrictions imposed by filesystems we need to
make sure the flag is only used when doing aligned IO.

The migration will do parallel IO to different regions of a file, so
we need to use more than one file descriptor. Those cannot be obtained
by duplicating (dup()) since duplicated file descriptors share the
file status flags, including O_DIRECT. If one migration channel does
unaligned IO while another sets O_DIRECT to do aligned IO, the
filesystem would fail the unaligned operation.

The add-fd QMP command along with the fdset code are specifically
designed to allow the user to pass a set of file descriptors with
different access flags into QEMU to be later fetched by code that
needs to alternate between those flags when doing IO.

Extend the fdset matching function to behave the same with the
O_DIRECT flag.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 monitor/fds.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/monitor/fds.c b/monitor/fds.c
index 9a28e4b72b..42bf3eb982 100644
--- a/monitor/fds.c
+++ b/monitor/fds.c
@@ -413,6 +413,12 @@ static bool monitor_fdset_flags_match(int flags, int 
fd_flags)
 
     if ((flags & O_ACCMODE) == (fd_flags & O_ACCMODE)) {
         match = true;
+
+#ifdef O_DIRECT
+        if ((flags & O_DIRECT) != (fd_flags & O_DIRECT)) {
+            match = false;
+        }
+#endif
     }
 
     return match;
-- 
2.35.3