block/null: Latency simulation by adding new option "latency-ns"

Aio context switch should just work because the requests will be
drained, so the scheduled timer(s) on the old context will be freed.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1427852740-24315-2-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/null.c

@@ -12,8 +12,11 @@
 
 #include "block/block_int.h"
 
+#define NULL_OPT_LATENCY "latency-ns"
+
 typedef struct {
     int64_t length;
+    int64_t latency_ns;
 } BDRVNullState;
 
 static QemuOptsList runtime_opts = {
@@ -30,6 +33,12 @@ static QemuOptsList runtime_opts = {
             .type = QEMU_OPT_SIZE,
             .help = "size of the null block",
         },
+        {
+            .name = NULL_OPT_LATENCY,
+            .type = QEMU_OPT_NUMBER,
+            .help = "nanoseconds (approximated) to wait "
+                    "before completing request",
+        },
         { /* end of list */ }
     },
 };
@@ -39,13 +48,20 @@ static int null_file_open(BlockDriverState *bs, QDict *options, int flags,
 {
     QemuOpts *opts;
     BDRVNullState *s = bs->opaque;
+    int ret = 0;
 
     opts = qemu_opts_create(&runtime_opts, NULL, 0, &error_abort);
     qemu_opts_absorb_qdict(opts, options, &error_abort);
     s->length =
         qemu_opt_get_size(opts, BLOCK_OPT_SIZE, 1 << 30);
+    s->latency_ns =
+        qemu_opt_get_number(opts, NULL_OPT_LATENCY, 0);
+    if (s->latency_ns < 0) {
+        error_setg(errp, "latency-ns is invalid");
+        ret = -EINVAL;
+    }
     qemu_opts_del(opts);
-    return 0;
+    return ret;
 }
 
 static void null_close(BlockDriverState *bs)
@@ -58,28 +74,40 @@ static int64_t null_getlength(BlockDriverState *bs)
     return s->length;
 }
 
+static coroutine_fn int null_co_common(BlockDriverState *bs)
+{
+    BDRVNullState *s = bs->opaque;
+
+    if (s->latency_ns) {
+        co_aio_sleep_ns(bdrv_get_aio_context(bs), QEMU_CLOCK_REALTIME,
+                        s->latency_ns);
+    }
+    return 0;
+}
+
 static coroutine_fn int null_co_readv(BlockDriverState *bs,
                                       int64_t sector_num, int nb_sectors,
                                       QEMUIOVector *qiov)
 {
-    return 0;
+    return null_co_common(bs);
 }
 
 static coroutine_fn int null_co_writev(BlockDriverState *bs,
                                        int64_t sector_num, int nb_sectors,
                                        QEMUIOVector *qiov)
 {
-    return 0;
+    return null_co_common(bs);
 }
 
 static coroutine_fn int null_co_flush(BlockDriverState *bs)
 {
-    return 0;
+    return null_co_common(bs);
 }
 
 typedef struct {
     BlockAIOCB common;
     QEMUBH *bh;
+    QEMUTimer timer;
 } NullAIOCB;
 
 static const AIOCBInfo null_aiocb_info = {
@@ -94,15 +122,33 @@ static void null_bh_cb(void *opaque)
     qemu_aio_unref(acb);
 }
 
+static void null_timer_cb(void *opaque)
+{
+    NullAIOCB *acb = opaque;
+    acb->common.cb(acb->common.opaque, 0);
+    timer_deinit(&acb->timer);
+    qemu_aio_unref(acb);
+}
+
 static inline BlockAIOCB *null_aio_common(BlockDriverState *bs,
                                           BlockCompletionFunc *cb,
                                           void *opaque)
 {
     NullAIOCB *acb;
+    BDRVNullState *s = bs->opaque;
 
     acb = qemu_aio_get(&null_aiocb_info, bs, cb, opaque);
+    /* Only emulate latency after vcpu is running. */
+    if (s->latency_ns) {
+        aio_timer_init(bdrv_get_aio_context(bs), &acb->timer,
+                       QEMU_CLOCK_REALTIME, SCALE_NS,
+                       null_timer_cb, acb);
+        timer_mod_ns(&acb->timer,
+                     qemu_clock_get_ns(QEMU_CLOCK_REALTIME) + s->latency_ns);
+    } else {
         acb->bh = aio_bh_new(bdrv_get_aio_context(bs), null_bh_cb, acb);
         qemu_bh_schedule(acb->bh);
+    }
     return &acb->common;
 }
 

qapi/block-core.json

@@ -1310,11 +1310,14 @@
 # Driver specific block device options for the null backend.
 #
 # @size:    #optional size of the device in bytes.
+# @latency-ns: #optional emulated latency (in nanoseconds) in processing
+#              requests. Default to zero which completes requests immediately.
+#              (Since 2.4)
 #
 # Since: 2.2
 ##
 { 'type': 'BlockdevOptionsNull',
-  'data': { '*size': 'int' } }
+  'data': { '*size': 'int', '*latency-ns': 'uint64' } }
 
 ##
 # @BlockdevOptionsVVFAT