[PATCH v3 10/14] migration/ram: Introduce 'fixed-ram' migration stream capability

[Nikolay Borisov]

Implement 'fixed-ram' feature. The core of the feature is to ensure that
each ram page of the migration stream has a specific offset in the
resulting migration stream. The reason why we'd want such behavior are
two fold:

 - When doing a 'fixed-ram' migration the resulting file will have a
   bounded size, since pages which are dirtied multiple times will
   always go to a fixed location in the file, rather than constantly
   being added to a sequential stream. This eliminates cases where a vm
   with, say, 1g of ram can result in a migration file that's 10s of
   Gbs, provided that the workload constantly redirties memory.

 - It paves the way to implement DIO-enabled save/restore of the
   migration stream as the pages are ensured to be written at aligned
   offsets.

The features requires changing the format. First, a bitmap is introduced
which tracks which pages have been written (i.e are dirtied) during
migration and subsequently it's being written in the resultin file,
again at a fixed location for every ramblock. Zero pages are ignored as
they'd be zero in the destination migration as well. With the changed
format data would look like the following:

|name len|name|used_len|pc*|bitmap_size|pages_offset|bitmap|pages|

* pc - refers to the page_size/mr->addr members, so newly added members
begin from "bitmap_size".

This layout is initialized during ram_save_setup so instead of having a
sequential stream of pages that follow the ramblock headers the dirty
pages for a ramblock follow its header. Since all pages have a fixed
location RAM_SAVE_FLAG_EOS is no longer generated on every migration
iteration but there is effectively a single RAM_SAVE_FLAG_EOS right at
the end.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 include/exec/ramblock.h |  7 +++
 migration/migration.c   | 51 +++++++++++++++++++++-
 migration/migration.h   |  1 +
 migration/ram.c         | 97 +++++++++++++++++++++++++++++++++--------
 migration/savevm.c      |  1 +
 qapi/migration.json     |  2 +-
 6 files changed, 138 insertions(+), 21 deletions(-)

diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h
index 6cbedf9e0c9a..30216c1a41d3 100644
--- a/include/exec/ramblock.h
+++ b/include/exec/ramblock.h
@@ -43,6 +43,13 @@ struct RAMBlock {
     size_t page_size;
     /* dirty bitmap used during migration */
     unsigned long *bmap;
+    /* shadow dirty bitmap used when migrating to a file */
+    unsigned long *shadow_bmap;
+    /* offset in the file pages belonging to this ramblock are saved, used
+     * only during migration to a file
+     */
+    off_t bitmap_offset;
+    uint64_t pages_offset;
     /* bitmap of already received pages in postcopy */
     unsigned long *receivedmap;
 
diff --git a/migration/migration.c b/migration/migration.c
index 11ceea340702..c7383845a5b4 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -165,7 +165,8 @@ INITIALIZE_MIGRATE_CAPS_SET(check_caps_background_snapshot,
     MIGRATION_CAPABILITY_XBZRLE,
     MIGRATION_CAPABILITY_X_COLO,
     MIGRATION_CAPABILITY_VALIDATE_UUID,
-    MIGRATION_CAPABILITY_ZERO_COPY_SEND);
+    MIGRATION_CAPABILITY_ZERO_COPY_SEND,
+    MIGRATION_CAPABILITY_FIXED_RAM);
 
 /* When we add fault tolerance, we could have several
    migrations at once.  For now we don't need to add
@@ -1326,6 +1327,27 @@ static bool migrate_caps_check(bool *cap_list,
     }
 #endif
 
+    if (cap_list[MIGRATION_CAPABILITY_FIXED_RAM]) {
+           if (cap_list[MIGRATION_CAPABILITY_MULTIFD]) {
+                   error_setg(errp, "Directly mapped memory incompatible with 
multifd");
+                   return false;
+           }
+
+           if (cap_list[MIGRATION_CAPABILITY_XBZRLE]) {
+                   error_setg(errp, "Directly mapped memory incompatible with 
xbzrle");
+                   return false;
+           }
+
+           if (cap_list[MIGRATION_CAPABILITY_COMPRESS]) {
+                   error_setg(errp, "Directly mapped memory incompatible with 
compression");
+                   return false;
+           }
+
+           if (cap_list[MIGRATION_CAPABILITY_POSTCOPY_RAM]) {
+                   error_setg(errp, "Directly mapped memory incompatible with 
postcopy ram");
+                   return false;
+           }
+    }
 
     /* incoming side only */
     if (runstate_check(RUN_STATE_INMIGRATE) &&
@@ -2630,6 +2652,11 @@ MultiFDCompression migrate_multifd_compression(void)
     return s->parameters.multifd_compression;
 }
 
+int migrate_fixed_ram(void)
+{
+    return 
migrate_get_current()->enabled_capabilities[MIGRATION_CAPABILITY_FIXED_RAM];
+}
+
 int migrate_multifd_zlib_level(void)
 {
     MigrationState *s;
@@ -4190,6 +4217,21 @@ static void *bg_migration_thread(void *opaque)
     return NULL;
 }
 
+static int
+migrate_check_fixed_ram(MigrationState *s, Error **errp)
+{
+    if (!s->enabled_capabilities[MIGRATION_CAPABILITY_FIXED_RAM])
+        return 0;
+
+    if (!qemu_file_is_seekable(s->to_dst_file)) {
+        error_setg(errp, "Directly mapped memory requires a seekable 
transport");
+        return -1;
+    }
+
+    return 0;
+}
+
+
 void migrate_fd_connect(MigrationState *s, Error *error_in)
 {
     Error *local_err = NULL;
@@ -4265,6 +4307,12 @@ void migrate_fd_connect(MigrationState *s, Error 
*error_in)
         return;
     }
 
+    if (migrate_check_fixed_ram(s, &local_err) < 0) {
+           migrate_fd_cleanup(s);
+           migrate_fd_error(s, local_err);
+           return;
+    }
+
     if (resume) {
         /* Wakeup the main migration thread to do the recovery */
         migrate_set_state(&s->state, MIGRATION_STATUS_POSTCOPY_PAUSED,
@@ -4398,6 +4446,7 @@ static Property migration_properties[] = {
     DEFINE_PROP_STRING("tls-authz", MigrationState, parameters.tls_authz),
 
     /* Migration capabilities */
+    DEFINE_PROP_MIG_CAP("x-fixed-ram", MIGRATION_CAPABILITY_FIXED_RAM),
     DEFINE_PROP_MIG_CAP("x-xbzrle", MIGRATION_CAPABILITY_XBZRLE),
     DEFINE_PROP_MIG_CAP("x-rdma-pin-all", MIGRATION_CAPABILITY_RDMA_PIN_ALL),
     DEFINE_PROP_MIG_CAP("x-auto-converge", MIGRATION_CAPABILITY_AUTO_CONVERGE),
diff --git a/migration/migration.h b/migration/migration.h
index 96f27aba2210..9aab1b16f407 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -410,6 +410,7 @@ bool migrate_zero_blocks(void);
 bool migrate_dirty_bitmaps(void);
 bool migrate_ignore_shared(void);
 bool migrate_validate_uuid(void);
+int migrate_fixed_ram(void);
 
 bool migrate_auto_converge(void);
 bool migrate_use_multifd(void);
diff --git a/migration/ram.c b/migration/ram.c
index dc1de9ddbc68..4f5ddaff356b 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1261,9 +1261,14 @@ static int save_zero_page_to_file(RAMState *rs, QEMUFile 
*file,
     int len = 0;
 
     if (buffer_is_zero(p, TARGET_PAGE_SIZE)) {
-        len += save_page_header(rs, file, block, offset | RAM_SAVE_FLAG_ZERO);
-        qemu_put_byte(file, 0);
-        len += 1;
+        if (migrate_fixed_ram()) {
+            /* for zero pages we don't need to do anything */
+            len = 1;
+        } else {
+            len += save_page_header(rs, file, block, offset | 
RAM_SAVE_FLAG_ZERO);
+            qemu_put_byte(file, 0);
+            len += 1;
+        }
         ram_release_page(block->idstr, offset);
     }
     return len;
@@ -1342,15 +1347,22 @@ static bool control_save_page(RAMState *rs, RAMBlock 
*block, ram_addr_t offset,
 static int save_normal_page(RAMState *rs, RAMBlock *block, ram_addr_t offset,
                             uint8_t *buf, bool async)
 {
-    ram_transferred_add(save_page_header(rs, rs->f, block,
-                                         offset | RAM_SAVE_FLAG_PAGE));
-    if (async) {
-        qemu_put_buffer_async(rs->f, buf, TARGET_PAGE_SIZE,
-                              migrate_release_ram() &&
-                              migration_in_postcopy());
-    } else {
-        qemu_put_buffer(rs->f, buf, TARGET_PAGE_SIZE);
-    }
+
+       if (migrate_fixed_ram()) {
+               qemu_put_buffer_at(rs->f, buf, TARGET_PAGE_SIZE,
+                                  block->pages_offset + offset);
+               set_bit(offset >> TARGET_PAGE_BITS, block->shadow_bmap);
+       } else {
+               ram_transferred_add(save_page_header(rs, rs->f, block,
+                                                    offset | 
RAM_SAVE_FLAG_PAGE));
+               if (async) {
+                       qemu_put_buffer_async(rs->f, buf, TARGET_PAGE_SIZE,
+                                             migrate_release_ram() &&
+                                             migration_in_postcopy());
+               } else {
+                       qemu_put_buffer(rs->f, buf, TARGET_PAGE_SIZE);
+               }
+       }
     ram_transferred_add(TARGET_PAGE_SIZE);
     ram_counters.normal++;
     return 1;
@@ -2683,6 +2695,8 @@ static void ram_save_cleanup(void *opaque)
         block->clear_bmap = NULL;
         g_free(block->bmap);
         block->bmap = NULL;
+        g_free(block->shadow_bmap);
+        block->shadow_bmap = NULL;
     }
 
     xbzrle_cleanup();
@@ -3044,6 +3058,7 @@ static void ram_list_init_bitmaps(void)
              */
             block->bmap = bitmap_new(pages);
             bitmap_set(block->bmap, 0, pages);
+            block->shadow_bmap = bitmap_new(block->used_length >> 
TARGET_PAGE_BITS);
             block->clear_bmap_shift = shift;
             block->clear_bmap = bitmap_new(clear_bmap_size(pages, shift));
         }
@@ -3226,12 +3241,34 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
             qemu_put_buffer(f, (uint8_t *)block->idstr, strlen(block->idstr));
             qemu_put_be64(f, block->used_length);
             if (migrate_postcopy_ram() && block->page_size !=
-                                          qemu_host_page_size) {
+                qemu_host_page_size) {
                 qemu_put_be64(f, block->page_size);
             }
             if (migrate_ignore_shared()) {
                 qemu_put_be64(f, block->mr->addr);
             }
+
+            if (migrate_fixed_ram()) {
+                long num_pages = block->used_length >> TARGET_PAGE_BITS;
+                long bitmap_size = BITS_TO_LONGS(num_pages) * sizeof(unsigned 
long);
+
+
+                /* Needed for external programs (think analyze-migration.py) */
+                qemu_put_be32(f, bitmap_size);
+
+                /*
+                 * Make pages offset aligned to TARGET_PAGE_SIZE to enable
+                 * DIO in the future. Also add 8 to account for the page offset
+                 * itself
+                 */
+                block->bitmap_offset = qemu_get_offset(f) + 8;
+                block->pages_offset = ROUND_UP(block->bitmap_offset +
+                                               bitmap_size, TARGET_PAGE_SIZE);
+                qemu_put_be64(f, block->pages_offset);
+
+                /* Now prepare offset for next ramblock */
+                qemu_set_offset(f, block->pages_offset + block->used_length, 
SEEK_SET);
+            }
         }
     }
 
@@ -3249,6 +3286,17 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
     return 0;
 }
 
+static void ram_save_shadow_bmap(QEMUFile *f)
+{
+    RAMBlock *block;
+
+    RAMBLOCK_FOREACH_MIGRATABLE(block) {
+        long num_pages = block->used_length >> TARGET_PAGE_BITS;
+        long bitmap_size = BITS_TO_LONGS(num_pages) * sizeof(unsigned long);
+        qemu_put_buffer_at(f, (uint8_t *)block->shadow_bmap, bitmap_size, 
block->bitmap_offset);
+    }
+}
+
 /**
  * ram_save_iterate: iterative stage for migration
  *
@@ -3358,9 +3406,15 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
             return ret;
         }
 
-        qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
-        qemu_fflush(f);
-        ram_transferred_add(8);
+       /*
+        * For fixed ram we don't want to pollute the migration stream with
+        * EOS flags.
+        */
+       if (!migrate_fixed_ram()) {
+            qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
+            qemu_fflush(f);
+            ram_transferred_add(8);
+       }
 
         ret = qemu_file_get_error(f);
     }
@@ -3405,7 +3459,10 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
             pages = ram_find_and_save_block(rs);
             /* no more blocks to sent */
             if (pages == 0) {
-                break;
+                if (migrate_fixed_ram()) {
+                    ram_save_shadow_bmap(f);
+                }
+            break;
             }
             if (pages < 0) {
                 ret = pages;
@@ -3428,8 +3485,10 @@ static int ram_save_complete(QEMUFile *f, void *opaque)
         return ret;
     }
 
-    qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
-    qemu_fflush(f);
+    if (!migrate_fixed_ram()) {
+        qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
+        qemu_fflush(f);
+    }
 
     return 0;
 }
diff --git a/migration/savevm.c b/migration/savevm.c
index 44a222888306..847a8bdfb6ce 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -240,6 +240,7 @@ static bool should_validate_capability(int capability)
     /* Validate only new capabilities to keep compatibility. */
     switch (capability) {
     case MIGRATION_CAPABILITY_X_IGNORE_SHARED:
+    case MIGRATION_CAPABILITY_FIXED_RAM:
         return true;
     default:
         return false;
diff --git a/qapi/migration.json b/qapi/migration.json
index 88ecf86ac876..6196617171e8 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -485,7 +485,7 @@
 ##
 { 'enum': 'MigrationCapability',
   'data': ['xbzrle', 'rdma-pin-all', 'auto-converge', 'zero-blocks',
-           'compress', 'events', 'postcopy-ram',
+           'compress', 'events', 'postcopy-ram', 'fixed-ram',
            { 'name': 'x-colo', 'features': [ 'unstable' ] },
            'release-ram',
            'block', 'return-path', 'pause-before-switchover', 'multifd',
-- 
2.34.1