[PATCH v2 2/4] multifd: document packet_len, next_packet_size

[Elena Ufimtseva]

next_packet_size name is a bit misleading, so add more comments
where its defined.
We send data in two chunks in multifd thread:
 - send the packet with normal (non-zero) guest pages offsets that are
   dirty.
   This uses the packet_len and we increment number of packets
   for this thread that are sent;
 - send the normal (non-zero) guest dirty pages themselves in iovs.
   The total size of the data pointed by all iovs for this chunk
   is next_packet_size. We do not increment the packet_num for this
   thread when sending actual pages;

When compression is enabled, next_packet_size is used to indicate
the size of the compressed buffer on source and destination.

Will be it helpful to rename it as data_size or dirty_data_size?

Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
---
 migration/multifd.h | 35 ++++++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/migration/multifd.h b/migration/multifd.h
index a835643b48..37da9b68c2 100644
--- a/migration/multifd.h
+++ b/migration/multifd.h
@@ -45,7 +45,13 @@ typedef struct {
     uint32_t pages_alloc;
     /* non zero pages */
     uint32_t normal_pages;
-    /* size of the next packet that contains pages */
+    /*
+     * amount of data to be sent to the destination
+     * that is calculated as
+     *  - number of the normal guest dirty pages * page_size in non
+     *    compression case;
+     *  - equals of the compressed data size to be received;
+     */
     uint32_t next_packet_size;
     uint64_t packet_num;
     uint64_t unused[4];    /* Reserved for future use */
@@ -79,11 +85,18 @@ typedef struct {
     QIOChannel *c;
     /* is the yank function registered */
     bool registered_yank;
-    /* packet allocated len */
+    /*
+     * allocated length of a packet to be transferred.
+     * It has a size of MultiFDPacket struct plus
+     * the size of the array of guest page offsets (page_count * page_size).
+     */
     uint32_t packet_len;
     /* guest page size */
     uint32_t page_size;
-    /* number of pages in a full packet */
+    /*
+     * maximum number of dirty pages in a full packet calculated as
+     * MULTIFD_PACKET_SIZE / qemu_target_page_size()
+     */
     uint32_t page_count;
     /* multifd flags for sending ram */
     int write_flags;
@@ -116,7 +129,13 @@ typedef struct {
 
     /* pointer to the packet */
     MultiFDPacket_t *packet;
-    /* size of the next packet that contains pages */
+    /*
+     * amount of data to be sent to the destination
+     * that is calculated as
+     *  - number of the normal guest dirty pages * page_size in non
+     *    compression case;
+     *  - equals of the compressed data size to be received;
+     */
     uint32_t next_packet_size;
     /* packets sent through this channel */
     uint64_t num_packets;
@@ -171,7 +190,13 @@ typedef struct {
 
     /* pointer to the packet */
     MultiFDPacket_t *packet;
-    /* size of the next packet that contains pages */
+    /*
+     * amount of data to be received by the destination
+     * that is calculated as
+     *  - number of the normal guest dirty pages * page_size in non
+     *    compression case;
+     *  - equals of the compressed data size to be received;
+     */
     uint32_t next_packet_size;
     /* packets sent through this channel */
     uint64_t num_packets;
-- 
2.34.1