[PATCH v4 05/10] mirror: implement mirror_change method

[Fiona Ebner <f.ebner@proxmox.com>]

which allows switching the @copy-mode from 'background' to
'write-blocking'.

This is useful for management applications, so they can start out in
background mode to avoid limiting guest write speed and switch to
active mode when certain criteria are fulfilled.

In presence of an iothread, the copy_mode member is now shared between
the iothread and the main thread, so turn accesses to it atomic.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

Changes in v4:
    * add comment about how copy_mode should be accessed
    * add GLOBAL_STATE_CODE annotation for mirror_change
    * use two spaces in between sentences in QAPI description
    * fix typo in QAPI description

 block/mirror.c       | 44 +++++++++++++++++++++++++++++++++++++++++---
 qapi/block-core.json | 13 ++++++++++++-
 2 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index 31da1526eb..4016d89253 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -55,6 +55,10 @@ typedef struct MirrorBlockJob {
     BlockMirrorBackingMode backing_mode;
     /* Whether the target image requires explicit zero-initialization */
     bool zero_target;
+    /*
+     * To be accesssed with atomics. Written only under the BQL (required by the
+     * current implementation of mirror_change()).
+     */
     MirrorCopyMode copy_mode;
     BlockdevOnError on_source_error, on_target_error;
     /* Set when the target is synced (dirty bitmap is clean, nothing
@@ -1075,7 +1079,7 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
                  */
                 job_transition_to_ready(&s->common.job);
             }
-            if (s->copy_mode != MIRROR_COPY_MODE_BACKGROUND) {
+            if (qatomic_read(&s->copy_mode) != MIRROR_COPY_MODE_BACKGROUND) {
                 s->actively_synced = true;
             }
 
@@ -1246,6 +1250,39 @@ static bool commit_active_cancel(Job *job, bool force)
     return force || !job_is_ready(job);
 }
 
+static void mirror_change(BlockJob *job, BlockJobChangeOptions *opts,
+                          Error **errp)
+{
+    MirrorBlockJob *s = container_of(job, MirrorBlockJob, common);
+    BlockJobChangeOptionsMirror *change_opts = &opts->u.mirror;
+    MirrorCopyMode current;
+
+    /*
+     * The implementation relies on the fact that copy_mode is only written
+     * under the BQL. Otherwise, further synchronization would be required.
+     */
+
+    GLOBAL_STATE_CODE();
+
+    if (qatomic_read(&s->copy_mode) == change_opts->copy_mode) {
+        return;
+    }
+
+    if (change_opts->copy_mode != MIRROR_COPY_MODE_WRITE_BLOCKING) {
+        error_setg(errp, "Change to copy mode '%s' is not implemented",
+                   MirrorCopyMode_str(change_opts->copy_mode));
+        return;
+    }
+
+    current = qatomic_cmpxchg(&s->copy_mode, MIRROR_COPY_MODE_BACKGROUND,
+                              change_opts->copy_mode);
+    if (current != MIRROR_COPY_MODE_BACKGROUND) {
+        error_setg(errp, "Expected current copy mode '%s', got '%s'",
+                   MirrorCopyMode_str(MIRROR_COPY_MODE_BACKGROUND),
+                   MirrorCopyMode_str(current));
+    }
+}
+
 static const BlockJobDriver mirror_job_driver = {
     .job_driver = {
         .instance_size          = sizeof(MirrorBlockJob),
@@ -1260,6 +1297,7 @@ static const BlockJobDriver mirror_job_driver = {
         .cancel                 = mirror_cancel,
     },
     .drained_poll           = mirror_drained_poll,
+    .change                 = mirror_change,
 };
 
 static const BlockJobDriver commit_active_job_driver = {
@@ -1467,7 +1505,7 @@ static bool should_copy_to_target(MirrorBDSOpaque *s)
 {
     return s->job && s->job->ret >= 0 &&
         !job_is_cancelled(&s->job->common.job) &&
-        s->job->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING;
+        qatomic_read(&s->job->copy_mode) == MIRROR_COPY_MODE_WRITE_BLOCKING;
 }
 
 static int coroutine_fn GRAPH_RDLOCK
@@ -1813,7 +1851,7 @@ static BlockJob *mirror_start_job(
     s->is_none_mode = is_none_mode;
     s->backing_mode = backing_mode;
     s->zero_target = zero_target;
-    s->copy_mode = copy_mode;
+    qatomic_set(&s->copy_mode, copy_mode);
     s->base = base;
     s->base_overlay = bdrv_find_overlay(bs, base);
     s->granularity = granularity;
diff --git a/qapi/block-core.json b/qapi/block-core.json
index c6f31a9399..6369207be2 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -3044,6 +3044,17 @@
 { 'command': 'block-job-finalize', 'data': { 'id': 'str' },
   'allow-preconfig': true }
 
+##
+# @BlockJobChangeOptionsMirror:
+#
+# @copy-mode: Switch to this copy mode.  Currently, only the switch
+#     from 'background' to 'write-blocking' is implemented.
+#
+# Since: 8.2
+##
+{ 'struct': 'BlockJobChangeOptionsMirror',
+  'data': { 'copy-mode' : 'MirrorCopyMode' } }
+
 ##
 # @BlockJobChangeOptions:
 #
@@ -3058,7 +3069,7 @@
 { 'union': 'BlockJobChangeOptions',
   'base': { 'id': 'str', 'type': 'JobType' },
   'discriminator': 'type',
-  'data': {} }
+  'data': { 'mirror': 'BlockJobChangeOptionsMirror' } }
 
 ##
 # @block-job-change:
-- 
2.39.2