Re: [PATCH v2 05/10] mirror: implement mirror_change method

[Vladimir Sementsov-Ogievskiy]

On 09.10.23 12:46, Fiona Ebner wrote:

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.

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

Changes in v2:
     * update QEMU version in QAPI
     * update indentation in QAPI (like in a937b6aa73 ("qapi: Reformat
       doc comments to conform to current conventions"))
     * drop drained section and disable dirty bitmap call. It's already
       disabled, because the bitmap is now attached to the filter and
       set in bdrv_mirror_top_do_write(). See the earlier patch
       "block/mirror: move dirty bitmap to filter"

  block/mirror.c       | 22 ++++++++++++++++++++++
  qapi/block-core.json | 13 ++++++++++++-
  2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/block/mirror.c b/block/mirror.c
index b84de56734..83aa4176c2 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1246,6 +1246,27 @@ 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;
+
+    if (s->copy_mode == change_opts->copy_mode) {
+        return;
+    }
+
+    if (s->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING) {
+        error_setg(errp, "Cannot switch away from copy mode 'write-blocking'");
+        return;
+    }
+
+    assert(s->copy_mode == MIRROR_COPY_MODE_BACKGROUND &&
+           change_opts->copy_mode == MIRROR_COPY_MODE_WRITE_BLOCKING);
+
+    s->copy_mode = MIRROR_COPY_MODE_WRITE_BLOCKING;
+}

So, s->copy_mode becomes shared between main thread and iothread.

We should either use mutex or atomic operations.

Note, that the only realization of .set_speed uses thread-safe API.

+
  static const BlockJobDriver mirror_job_driver = {
      .job_driver = {
          .instance_size          = sizeof(MirrorBlockJob),
@@ -1260,6 +1281,7 @@ static const BlockJobDriver mirror_job_driver = {
          .cancel                 = mirror_cancel,
      },
      .drained_poll           = mirror_drained_poll,
+    .change                 = mirror_change,
  };

static const BlockJobDriver commit_active_job_driver = {

diff --git a/qapi/block-core.json b/qapi/block-core.json
index c6f31a9399..01427c259a 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. Currenlty, 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:

--
Best regards,
Vladimir