migration/multifd: Move header prepare/fill into send_prepare()

This patch redefines the interfacing of ->send_prepare().  It further
simplifies multifd_send_thread() especially on zero copy.

Now with the new interface, we require the hook to do all the work for
preparing the IOVs to send.  After it's completed, the IOVs should be ready
to be dumped into the specific multifd QIOChannel later.

So now the API looks like:

  p->pages ----------->  send_prepare() -------------> IOVs

This also prepares for the case where the input can be extended to even not
any p->pages.  But that's for later.

This patch will achieve similar goal of what Fabiano used to propose here:

https://lore.kernel.org/r/20240126221943.26628-1-farosas@suse.de

However the send() interface may not be necessary.  I'm boldly attaching a
"Co-developed-by" for Fabiano.

Co-developed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240202102857.110210-14-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/multifd-zlib.c

@@ -123,6 +123,8 @@ static int zlib_send_prepare(MultiFDSendParams *p, Error **errp)
     int ret;
     uint32_t i;
 
+    multifd_send_prepare_header(p);
+
     for (i = 0; i < pages->num; i++) {
         uint32_t available = z->zbuff_len - out_size;
         int flush = Z_NO_FLUSH;
@@ -172,6 +174,8 @@ static int zlib_send_prepare(MultiFDSendParams *p, Error **errp)
     p->next_packet_size = out_size;
     p->flags |= MULTIFD_FLAG_ZLIB;
 
+    multifd_send_fill_packet(p);
+
     return 0;
 }
 

migration/multifd-zstd.c

@@ -118,6 +118,8 @@ static int zstd_send_prepare(MultiFDSendParams *p, Error **errp)
     int ret;
     uint32_t i;
 
+    multifd_send_prepare_header(p);
+
     z->out.dst = z->zbuff;
     z->out.size = z->zbuff_len;
     z->out.pos = 0;
@@ -161,6 +163,8 @@ static int zstd_send_prepare(MultiFDSendParams *p, Error **errp)
     p->next_packet_size = z->out.pos;
     p->flags |= MULTIFD_FLAG_ZSTD;
 
+    multifd_send_fill_packet(p);
+
     return 0;
 }
 

migration/multifd.c

@@ -50,15 +50,15 @@ typedef struct {
 /**
  * nocomp_send_setup: setup send side
  *
- * For no compression this function does nothing.
- *
- * Returns 0 for success or -1 for error
- *
  * @p: Params for the channel that we are using
  * @errp: pointer to an error
  */
 static int nocomp_send_setup(MultiFDSendParams *p, Error **errp)
 {
+    if (migrate_zero_copy_send()) {
+        p->write_flags |= QIO_CHANNEL_WRITE_FLAG_ZERO_COPY;
+    }
+
     return 0;
 }
 
@@ -88,7 +88,17 @@ static void nocomp_send_cleanup(MultiFDSendParams *p, Error **errp)
  */
 static int nocomp_send_prepare(MultiFDSendParams *p, Error **errp)
 {
+    bool use_zero_copy_send = migrate_zero_copy_send();
     MultiFDPages_t *pages = p->pages;
+    int ret;
+
+    if (!use_zero_copy_send) {
+        /*
+         * Only !zerocopy needs the header in IOV; zerocopy will
+         * send it separately.
+         */
+        multifd_send_prepare_header(p);
+    }
 
     for (int i = 0; i < pages->num; i++) {
         p->iov[p->iovs_num].iov_base = pages->block->host + pages->offset[i];
@@ -98,6 +108,18 @@ static int nocomp_send_prepare(MultiFDSendParams *p, Error **errp)
 
     p->next_packet_size = pages->num * p->page_size;
     p->flags |= MULTIFD_FLAG_NOCOMP;
+
+    multifd_send_fill_packet(p);
+
+    if (use_zero_copy_send) {
+        /* Send header first, without zerocopy */
+        ret = qio_channel_write_all(p->c, (void *)p->packet,
+                                    p->packet_len, errp);
+        if (ret != 0) {
+            return -1;
+        }
+    }
+
     return 0;
 }
 
@@ -266,7 +288,7 @@ static void multifd_pages_clear(MultiFDPages_t *pages)
     g_free(pages);
 }
 
-static void multifd_send_fill_packet(MultiFDSendParams *p)
+void multifd_send_fill_packet(MultiFDSendParams *p)
 {
     MultiFDPacket_t *packet = p->packet;
     MultiFDPages_t *pages = p->pages;
@@ -688,7 +710,6 @@ static void *multifd_send_thread(void *opaque)
     MigrationThread *thread = NULL;
     Error *local_err = NULL;
     int ret = 0;
-    bool use_zero_copy_send = migrate_zero_copy_send();
 
     thread = migration_threads_add(p->name, qemu_get_thread_id());
 
@@ -713,15 +734,6 @@ static void *multifd_send_thread(void *opaque)
             MultiFDPages_t *pages = p->pages;
 
             p->iovs_num = 0;
-
-            if (!use_zero_copy_send) {
-                /*
-                 * Only !zerocopy needs the header in IOV; zerocopy will
-                 * send it separately.
-                 */
-                multifd_send_prepare_header(p);
-            }
-
             assert(pages->num);
 
             ret = multifd_send_state->ops->send_prepare(p, &local_err);
@@ -730,17 +742,6 @@ static void *multifd_send_thread(void *opaque)
                 break;
             }
 
-            multifd_send_fill_packet(p);
-
-            if (use_zero_copy_send) {
-                /* Send header first, without zerocopy */
-                ret = qio_channel_write_all(p->c, (void *)p->packet,
-                                            p->packet_len, &local_err);
-                if (ret != 0) {
-                    break;
-                }
-            }
-
             ret = qio_channel_writev_full_all(p->c, p->iov, p->iovs_num, NULL,
                                               0, p->write_flags, &local_err);
             if (ret != 0) {
@@ -945,13 +946,7 @@ int multifd_save_setup(Error **errp)
         p->iov = g_new0(struct iovec, page_count + 1);
         p->page_size = qemu_target_page_size();
         p->page_count = page_count;
-
-        if (migrate_zero_copy_send()) {
-            p->write_flags = QIO_CHANNEL_WRITE_FLAG_ZERO_COPY;
-        } else {
-            p->write_flags = 0;
-        }
-
+        p->write_flags = 0;
         multifd_new_send_channel_create(p);
     }
 

migration/multifd.h

@@ -208,6 +208,7 @@ typedef struct {
 } MultiFDMethods;
 
 void multifd_register_ops(int method, MultiFDMethods *ops);
+void multifd_send_fill_packet(MultiFDSendParams *p);
 
 static inline void multifd_send_prepare_header(MultiFDSendParams *p)
 {