[PATCH v2] linux-aio: add IO_CMD_FDSYNC command support

[Prasad Pandit <ppandit@redhat.com>]

From: Prasad Pandit <pjp@fedoraproject.org>

Libaio defines IO_CMD_FDSYNC command to sync all outstanding
asynchronous I/O operations, by flushing out file data to the
disk storage.

Enable linux-aio to submit such aio request. This helps to
reduce latency induced via pthread_create calls by
thread-pool (aio=threads).

Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
---
 block/file-posix.c | 12 ++++++++++++
 block/linux-aio.c  |  5 ++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

v2: if IO_CMD_FDSYNC is not supported by the kernel,
    fallback on thread-pool flush.
  -> https://lists.nongnu.org/archive/html/qemu-devel/2024-03/msg01986.html

diff --git a/block/file-posix.c b/block/file-posix.c
index 35684f7e21..4f2195d01d 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2599,6 +2599,18 @@ static int coroutine_fn raw_co_flush_to_disk(BlockDriverState *bs)
     if (raw_check_linux_io_uring(s)) {
         return luring_co_submit(bs, s->fd, 0, NULL, QEMU_AIO_FLUSH);
     }
+#endif
+#ifdef CONFIG_LINUX_AIO
+    if (raw_check_linux_aio(s)) {
+        ret = laio_co_submit(s->fd, 0, NULL, QEMU_AIO_FLUSH, 0);
+        if (ret >= 0) {
+            /*
+             * if AIO_FLUSH is supported return
+             * else fallback on thread-pool flush.
+             */
+            return ret;
+        }
+    }
 #endif
     return raw_thread_pool_submit(handle_aiocb_flush, &acb);
 }
diff --git a/block/linux-aio.c b/block/linux-aio.c
index ec05d946f3..d940d029e3 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -384,6 +384,9 @@ static int laio_do_submit(int fd, struct qemu_laiocb *laiocb, off_t offset,
     case QEMU_AIO_READ:
         io_prep_preadv(iocbs, fd, qiov->iov, qiov->niov, offset);
         break;
+    case QEMU_AIO_FLUSH:
+        io_prep_fdsync(iocbs, fd);
+        break;
     /* Currently Linux kernel does not support other operations */
     default:
         fprintf(stderr, "%s: invalid AIO request type 0x%x.\n",
@@ -412,7 +415,7 @@ int coroutine_fn laio_co_submit(int fd, uint64_t offset, QEMUIOVector *qiov,
     AioContext *ctx = qemu_get_current_aio_context();
     struct qemu_laiocb laiocb = {
         .co         = qemu_coroutine_self(),
-        .nbytes     = qiov->size,
+        .nbytes     = qiov ? qiov->size : 0,
         .ctx        = aio_get_linux_aio(ctx),
         .ret        = -EINPROGRESS,
         .is_read    = (type == QEMU_AIO_READ),
-- 
2.44.0