During post-copy migration (once it actually switches to post-copy mode)
dirty memory pages are continued to be migrated iteratively, while the
destination can explicitly request a specific page to be migrated before
the iterative process gets to it (which happens when a guest wants to
read a page that was not migrated yet). Without the postcopy-preempt
capability enabled such pages need to wait until all other pages already
queued are transferred. Enabling this capability will instruct the
hypervisor to create a separate migration channel for explicitly
requested pages so that they can preempt the queue.

This is enabled for all post-copy migration as long as the capability is
supported on both sides of migration.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
---
 src/qemu/qemu_migration.c        | 11 +++++++----
 src/qemu/qemu_migration_params.c |  2 ++
 src/qemu/qemu_migration_params.h |  1 +
 3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/qemu/qemu_migration.c b/src/qemu/qemu_migration.c
index 5504119079..25dc16a9e9 100644
--- a/src/qemu/qemu_migration.c
+++ b/src/qemu/qemu_migration.c
@@ -5203,17 +5203,20 @@ qemuMigrationSrcPerformNative(virQEMUDriver *driver,
             return -1;
         }
 
-        if (flags & VIR_MIGRATE_PARALLEL)
+        /* multi-fd and postcopy-preempt require QEMU to connect to the
+         * destination itself */
+        if (flags & (VIR_MIGRATE_PARALLEL | VIR_MIGRATE_POSTCOPY))
             spec.destType = MIGRATION_DEST_SOCKET;
         else
             spec.destType = MIGRATION_DEST_CONNECT_SOCKET;
 
         spec.dest.socket.path = uribits->path;
     } else {
-        /* RDMA and multi-fd migration requires QEMU to connect to the destination
-         * itself.
+        /* RDMA, multi-fd, and postcopy-preempt migration require QEMU to
+         * connect to the destination itself.
          */
-        if (STREQ(uribits->scheme, "rdma") || (flags & VIR_MIGRATE_PARALLEL))
+        if (STREQ(uribits->scheme, "rdma") ||
+            flags & (VIR_MIGRATE_PARALLEL | VIR_MIGRATE_POSTCOPY))
             spec.destType = MIGRATION_DEST_HOST;
         else
             spec.destType = MIGRATION_DEST_CONNECT_HOST;
diff --git a/src/qemu/qemu_migration_params.c b/src/qemu/qemu_migration_params.c
index 133f1f762d..9a399c7162 100644
--- a/src/qemu/qemu_migration_params.c
+++ b/src/qemu/qemu_migration_params.c
@@ -104,6 +104,7 @@ VIR_ENUM_IMPL(qemuMigrationCapability,
               "dirty-bitmaps",
               "return-path",
               "zero-copy-send",
+              "postcopy-preempt",
 );
 
 
@@ -197,6 +198,7 @@ static const qemuMigrationParamsFlagMapItem qemuMigrationParamsFlagMap[] = {
     {.match = QEMU_MIGRATION_FLAG_REQUIRED,
      .flag = VIR_MIGRATE_POSTCOPY,
      .cap = QEMU_MIGRATION_CAP_POSTCOPY,
+     .optional = QEMU_MIGRATION_CAP_POSTCOPY_PREEMPT,
      .party = QEMU_MIGRATION_SOURCE | QEMU_MIGRATION_DESTINATION},
 
     {.match = QEMU_MIGRATION_FLAG_REQUIRED,
diff --git a/src/qemu/qemu_migration_params.h b/src/qemu/qemu_migration_params.h
index 115d7bc597..91bc6792cd 100644
--- a/src/qemu/qemu_migration_params.h
+++ b/src/qemu/qemu_migration_params.h
@@ -40,6 +40,7 @@ typedef enum {
     QEMU_MIGRATION_CAP_BLOCK_DIRTY_BITMAPS,
     QEMU_MIGRATION_CAP_RETURN_PATH,
     QEMU_MIGRATION_CAP_ZERO_COPY_SEND,
+    QEMU_MIGRATION_CAP_POSTCOPY_PREEMPT,
 
     QEMU_MIGRATION_CAP_LAST
 } qemuMigrationCapability;
-- 
2.43.0
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On Mon, Jan 08, 2024 at 18:06:41 +0100, Jiri Denemark wrote:
> During post-copy migration (once it actually switches to post-copy mode)
> dirty memory pages are continued to be migrated iteratively, while the
> destination can explicitly request a specific page to be migrated before
> the iterative process gets to it (which happens when a guest wants to
> read a page that was not migrated yet). Without the postcopy-preempt
> capability enabled such pages need to wait until all other pages already
> queued are transferred. Enabling this capability will instruct the
> hypervisor to create a separate migration channel for explicitly
> requested pages so that they can preempt the queue.

You should state more explicitly that this requires the change in
connection method from MIGRATION_DEST_CONNECT_SOCKET to MIGRATION_DEST_SOCKET

> 
> This is enabled for all post-copy migration as long as the capability is
> supported on both sides of migration.
> 
> Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
> ---
>  src/qemu/qemu_migration.c        | 11 +++++++----
>  src/qemu/qemu_migration_params.c |  2 ++
>  src/qemu/qemu_migration_params.h |  1 +
>  3 files changed, 10 insertions(+), 4 deletions(-)

[...]

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
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