[PATCH v8 1/3] multifd: Create property multifd-flush-after-each-section

[Juan Quintela]

We used to flush all channels at the end of each RAM section
sent.  That is not needed, so preparing to only flush after a full
iteration through all the RAM.

Default value of the property is false.  But we return "true" in
migrate_multifd_flush_after_each_section() until we implement the code
in following patches.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

---

Rename each-iteration to after-each-section
Rename multifd-sync-after-each-section to
       multifd-flush-after-each-section
---
 hw/core/machine.c     |  1 +
 migration/migration.c | 13 +++++++++++++
 migration/migration.h | 13 +++++++++++++
 3 files changed, 27 insertions(+)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 2ce97a5d3b..32bd9277b3 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -60,6 +60,7 @@ const size_t hw_compat_7_1_len = G_N_ELEMENTS(hw_compat_7_1);
 GlobalProperty hw_compat_7_0[] = {
     { "arm-gicv3-common", "force-8-bit-prio", "on" },
     { "nvme-ns", "eui64-default", "on"},
+    { "migration", "multifd-flush-after-each-section", "on"},
 };
 const size_t hw_compat_7_0_len = G_N_ELEMENTS(hw_compat_7_0);
 
diff --git a/migration/migration.c b/migration/migration.c
index d91fe9fd86..4b94360f05 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2697,6 +2697,17 @@ bool migrate_use_multifd(void)
     return s->capabilities[MIGRATION_CAPABILITY_MULTIFD];
 }
 
+bool migrate_multifd_flush_after_each_section(void)
+{
+    MigrationState *s = migrate_get_current();
+
+    /*
+     * Until the patch that remove this comment, we always return that
+     * the property is enabled.
+     */
+    return true || s->multifd_flush_after_each_section;
+}
+
 bool migrate_pause_before_switchover(void)
 {
     MigrationState *s;
@@ -4448,6 +4459,8 @@ static Property migration_properties[] = {
                      send_section_footer, true),
     DEFINE_PROP_BOOL("decompress-error-check", MigrationState,
                       decompress_error_check, true),
+    DEFINE_PROP_BOOL("multifd-flush-after-each-section", MigrationState,
+                      multifd_flush_after_each_section, true),
     DEFINE_PROP_UINT8("x-clear-bitmap-shift", MigrationState,
                       clear_bitmap_shift, CLEAR_BITMAP_SHIFT_DEFAULT),
     DEFINE_PROP_BOOL("x-preempt-pre-7-2", MigrationState,
diff --git a/migration/migration.h b/migration/migration.h
index 04e0860b4e..50b4006e93 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -404,6 +404,18 @@ struct MigrationState {
      */
     bool preempt_pre_7_2;
 
+    /*
+     * flush every channel after each section sent.
+     *
+     * This assures that we can't mix pages from one iteration through
+     * ram pages with pages for the following iteration.  We really
+     * only need to do this flush after we have go through all the
+     * dirty pages.  For historical reasons, we do that after each
+     * section.  This is suboptimal (we flush too many times).
+     * Default value is false.  Setting this property has no effect
+     * until the patch that removes this comment.  (since 8.1)
+     */
+    bool multifd_flush_after_each_section;
     /*
      * This decides the size of guest memory chunk that will be used
      * to track dirty bitmap clearing.  The size of memory chunk will
@@ -463,6 +475,7 @@ int migrate_multifd_channels(void);
 MultiFDCompression migrate_multifd_compression(void);
 int migrate_multifd_zlib_level(void);
 int migrate_multifd_zstd_level(void);
+bool migrate_multifd_flush_after_each_section(void);
 
 #ifdef CONFIG_LINUX
 bool migrate_use_zero_copy_send(void);
-- 
2.40.0