linux aio: some comments

Add some notes about Linux AIO explaining why we don't use AIO in
some situations.

Signed-off-by: Frediano Ziglio <freddy77@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/raw-posix.c

@@ -236,6 +236,10 @@ static int raw_open_common(BlockDriverState *bs, const char *filename,
     }
 
 #ifdef CONFIG_LINUX_AIO
+    /*
+     * Currently Linux do AIO only for files opened with O_DIRECT
+     * specified so check NOCACHE flag too
+     */
     if ((bdrv_flags & (BDRV_O_NOCACHE|BDRV_O_NATIVE_AIO)) ==
                       (BDRV_O_NOCACHE|BDRV_O_NATIVE_AIO)) {
 

linux-aio.c

@@ -181,6 +181,7 @@ BlockDriverAIOCB *laio_submit(BlockDriverState *bs, void *aio_ctx, int fd,
     case QEMU_AIO_READ:
         io_prep_preadv(iocbs, fd, qiov->iov, qiov->niov, offset);
 	break;
+    /* Currently Linux kernel does not support other operations */
     default:
         fprintf(stderr, "%s: invalid AIO request type 0x%x.\n",
                         __func__, type);