From: Stefano Garzarella <sgarzare@redhat.com>

Commit d7ddd0a161 ("linux-aio: limit the batch size using
`aio-max-batch` parameter") added a way to limit the batch size
of Linux AIO backend for the entire AIO context.

The same AIO context can be shared by multiple devices, so
latency-sensitive devices may want to limit the batch size even
more to avoid increasing latency.

For this reason we add the `aio-max-batch` option to the file
backend, which will be used by the next commits to limit the size of
batches including requests generated by this device.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20211026162346.253081-2-sgarzare@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 qapi/block-core.json | 7 +++++++
 block/file-posix.c   | 9 +++++++++
 2 files changed, 16 insertions(+)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index ce2c1352cb..ea36e0038c 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2939,6 +2939,12 @@
 #              for this device (default: none, forward the commands via SG_IO;
 #              since 2.11)
 # @aio: AIO backend (default: threads) (since: 2.8)
+# @aio-max-batch: maximum number of requests to batch together into a single
+#                 submission in the AIO backend. The smallest value between
+#                 this and the aio-max-batch value of the IOThread object is
+#                 chosen.
+#                 0 means that the AIO backend will handle it automatically.
+#                 (default: 0, since 6.2)
 # @locking: whether to enable file locking. If set to 'auto', only enable
 #           when Open File Descriptor (OFD) locking API is available
 #           (default: auto, since 2.10)
@@ -2968,6 +2974,7 @@
             '*pr-manager': 'str',
             '*locking': 'OnOffAuto',
             '*aio': 'BlockdevAioOptions',
+            '*aio-max-batch': 'int',
             '*drop-cache': {'type': 'bool',
                             'if': 'CONFIG_LINUX'},
             '*x-check-cache-dropped': { 'type': 'bool',
diff --git a/block/file-posix.c b/block/file-posix.c
index 6def2a4cba..7a289a9481 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -150,6 +150,8 @@ typedef struct BDRVRawState {
     uint64_t locked_perm;
     uint64_t locked_shared_perm;
 
+    uint64_t aio_max_batch;
+
     int perm_change_fd;
     int perm_change_flags;
     BDRVReopenState *reopen_state;
@@ -530,6 +532,11 @@ static QemuOptsList raw_runtime_opts = {
             .type = QEMU_OPT_STRING,
             .help = "host AIO implementation (threads, native, io_uring)",
         },
+        {
+            .name = "aio-max-batch",
+            .type = QEMU_OPT_NUMBER,
+            .help = "AIO max batch size (0 = auto handled by AIO backend, default: 0)",
+        },
         {
             .name = "locking",
             .type = QEMU_OPT_STRING,
@@ -609,6 +616,8 @@ static int raw_open_common(BlockDriverState *bs, QDict *options,
     s->use_linux_io_uring = (aio == BLOCKDEV_AIO_OPTIONS_IO_URING);
 #endif
 
+    s->aio_max_batch = qemu_opt_get_number(opts, "aio-max-batch", 0);
+
     locking = qapi_enum_parse(&OnOffAuto_lookup,
                               qemu_opt_get(opts, "locking"),
                               ON_OFF_AUTO_AUTO, &local_err);
-- 
2.31.1