[Qemu-devel] [RFC PATCH] migration: discard RAMBlocks of type ram_device

["Cédric Le Goater" <clg@kaod.org>]

Here is some context for this strange change request.

On the POWER9 processor, the XIVE interrupt controller can control
interrupt sources using MMIO to trigger events, to EOI or to turn off
the sources. Priority management and interrupt acknowledgment is also
controlled by MMIO in the presenter subengine.

These MMIO regions are exposed to guests in QEMU with a set of 'ram
device' memory mappings, similarly to VFIO, and the VMAs are populated
dynamically with the appropriate pages using a fault handler.

But, these regions are an issue for migration. We need to discard the
associated RAMBlocks from the RAM state on the source VM and let the
destination VM rebuild the memory mappings on the new host in the
post_load() operation just before resuming the system.

This is the goal of the following proposal. Does it make sense ? It
seems to be working enough to migrate a running guest but there might
be a better, more subtle, approach.

Thanks,

C.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 migration/ram.c | 42 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 40 insertions(+), 2 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 0e90efa09236..6404ccd046d8 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -780,6 +780,10 @@ unsigned long migration_bitmap_find_dirty(RAMState *rs, RAMBlock *rb,
     unsigned long *bitmap = rb->bmap;
     unsigned long next;
 
+    if (memory_region_is_ram_device(rb->mr)) {
+        return size;
+    }
+
     if (rs->ram_bulk_stage && start > 0) {
         next = start + 1;
     } else {
@@ -826,6 +830,9 @@ uint64_t ram_pagesize_summary(void)
     uint64_t summary = 0;
 
     RAMBLOCK_FOREACH(block) {
+        if (memory_region_is_ram_device(block->mr)) {
+            continue;
+        }
         summary |= block->page_size;
     }
 
@@ -850,6 +857,9 @@ static void migration_bitmap_sync(RAMState *rs)
     qemu_mutex_lock(&rs->bitmap_mutex);
     rcu_read_lock();
     RAMBLOCK_FOREACH(block) {
+        if (memory_region_is_ram_device(block->mr)) {
+            continue;
+        }
         migration_bitmap_sync_range(rs, block, 0, block->used_length);
     }
     rcu_read_unlock();
@@ -1499,6 +1509,10 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
     size_t pagesize_bits =
         qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
 
+    if (memory_region_is_ram_device(pss->block->mr)) {
+        return 0;
+    }
+
     do {
         tmppages = ram_save_target_page(rs, pss, last_stage);
         if (tmppages < 0) {
@@ -1588,6 +1602,9 @@ uint64_t ram_bytes_total(void)
 
     rcu_read_lock();
     RAMBLOCK_FOREACH(block) {
+        if (memory_region_is_ram_device(block->mr)) {
+            continue;
+        }
         total += block->used_length;
     }
     rcu_read_unlock();
@@ -1643,6 +1660,9 @@ static void ram_save_cleanup(void *opaque)
     memory_global_dirty_log_stop();
 
     QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
+        if (memory_region_is_ram_device(block->mr)) {
+            continue;
+        }
         g_free(block->bmap);
         block->bmap = NULL;
         g_free(block->unsentmap);
@@ -1710,6 +1730,9 @@ void ram_postcopy_migrated_memory_release(MigrationState *ms)
         unsigned long range = block->used_length >> TARGET_PAGE_BITS;
         unsigned long run_start = find_next_zero_bit(bitmap, range, 0);
 
+        if (memory_region_is_ram_device(block->mr)) {
+            continue;
+        }
         while (run_start < range) {
             unsigned long run_end = find_next_bit(bitmap, range, run_start + 1);
             ram_discard_range(block->idstr, run_start << TARGET_PAGE_BITS,
@@ -1784,8 +1807,13 @@ static int postcopy_each_ram_send_discard(MigrationState *ms)
     int ret;
 
     RAMBLOCK_FOREACH(block) {
-        PostcopyDiscardState *pds =
-            postcopy_discard_send_init(ms, block->idstr);
+        PostcopyDiscardState *pds;
+
+        if (memory_region_is_ram_device(block->mr)) {
+            continue;
+        }
+
+        pds = postcopy_discard_send_init(ms, block->idstr);
 
         /*
          * Postcopy sends chunks of bitmap over the wire, but it
@@ -1996,6 +2024,10 @@ int ram_postcopy_send_discard_bitmap(MigrationState *ms)
         unsigned long *bitmap = block->bmap;
         unsigned long *unsentmap = block->unsentmap;
 
+        if (memory_region_is_ram_device(block->mr)) {
+            continue;
+        }
+
         if (!unsentmap) {
             /* We don't have a safe way to resize the sentmap, so
              * if the bitmap was resized it will be NULL at this
@@ -2151,6 +2183,9 @@ static void ram_list_init_bitmaps(void)
     /* Skip setting bitmap if there is no RAM */
     if (ram_bytes_total()) {
         QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
+            if (memory_region_is_ram_device(block->mr)) {
+                continue;
+            }
             pages = block->max_length >> TARGET_PAGE_BITS;
             block->bmap = bitmap_new(pages);
             bitmap_set(block->bmap, 0, pages);
@@ -2227,6 +2262,9 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
     qemu_put_be64(f, ram_bytes_total() | RAM_SAVE_FLAG_MEM_SIZE);
 
     RAMBLOCK_FOREACH(block) {
+        if (memory_region_is_ram_device(block->mr)) {
+            continue;
+        }
         qemu_put_byte(f, strlen(block->idstr));
         qemu_put_buffer(f, (uint8_t *)block->idstr, strlen(block->idstr));
         qemu_put_be64(f, block->used_length);
-- 
2.13.6