memory: add support getting and using a dirty bitmap copy.

This patch adds support for getting and using a local copy of the dirty
bitmap.

memory_region_snapshot_and_clear_dirty() will create a snapshot of the
dirty bitmap for the specified range, clear the dirty bitmap and return
the copy.  The returned bitmap can be a bit larger than requested, the
range is expanded so the code can copy unsigned longs from the bitmap
and avoid atomic bit update operations.

memory_region_snapshot_get_dirty() will return the dirty status of
pages, pretty much like memory_region_get_dirty(), but using the copy
returned by memory_region_copy_and_clear_dirty().

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170421091632.30900-3-kraxel@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

exec.c

@@ -223,6 +223,12 @@ struct CPUAddressSpace {
     MemoryListener tcg_as_listener;
 };
 
+struct DirtyBitmapSnapshot {
+    ram_addr_t start;
+    ram_addr_t end;
+    unsigned long dirty[];
+};
+
 #endif
 
 #if !defined(CONFIG_USER_ONLY)
@@ -1061,6 +1067,75 @@ bool cpu_physical_memory_test_and_clear_dirty(ram_addr_t start,
     return dirty;
 }
 
+DirtyBitmapSnapshot *cpu_physical_memory_snapshot_and_clear_dirty
+     (ram_addr_t start, ram_addr_t length, unsigned client)
+{
+    DirtyMemoryBlocks *blocks;
+    unsigned long align = 1UL << (TARGET_PAGE_BITS + BITS_PER_LEVEL);
+    ram_addr_t first = QEMU_ALIGN_DOWN(start, align);
+    ram_addr_t last  = QEMU_ALIGN_UP(start + length, align);
+    DirtyBitmapSnapshot *snap;
+    unsigned long page, end, dest;
+
+    snap = g_malloc0(sizeof(*snap) +
+                     ((last - first) >> (TARGET_PAGE_BITS + 3)));
+    snap->start = first;
+    snap->end   = last;
+
+    page = first >> TARGET_PAGE_BITS;
+    end  = last  >> TARGET_PAGE_BITS;
+    dest = 0;
+
+    rcu_read_lock();
+
+    blocks = atomic_rcu_read(&ram_list.dirty_memory[client]);
+
+    while (page < end) {
+        unsigned long idx = page / DIRTY_MEMORY_BLOCK_SIZE;
+        unsigned long offset = page % DIRTY_MEMORY_BLOCK_SIZE;
+        unsigned long num = MIN(end - page, DIRTY_MEMORY_BLOCK_SIZE - offset);
+
+        assert(QEMU_IS_ALIGNED(offset, (1 << BITS_PER_LEVEL)));
+        assert(QEMU_IS_ALIGNED(num,    (1 << BITS_PER_LEVEL)));
+        offset >>= BITS_PER_LEVEL;
+
+        bitmap_copy_and_clear_atomic(snap->dirty + dest,
+                                     blocks->blocks[idx] + offset,
+                                     num);
+        page += num;
+        dest += num >> BITS_PER_LEVEL;
+    }
+
+    rcu_read_unlock();
+
+    if (tcg_enabled()) {
+        tlb_reset_dirty_range_all(start, length);
+    }
+
+    return snap;
+}
+
+bool cpu_physical_memory_snapshot_get_dirty(DirtyBitmapSnapshot *snap,
+                                            ram_addr_t start,
+                                            ram_addr_t length)
+{
+    unsigned long page, end;
+
+    assert(start >= snap->start);
+    assert(start + length <= snap->end);
+
+    end = TARGET_PAGE_ALIGN(start + length - snap->start) >> TARGET_PAGE_BITS;
+    page = (start - snap->start) >> TARGET_PAGE_BITS;
+
+    while (page < end) {
+        if (test_bit(page, snap->dirty)) {
+            return true;
+        }
+        page++;
+    }
+    return false;
+}
+
 /* Called from RCU critical section */
 hwaddr memory_region_section_get_iotlb(CPUState *cpu,
                                        MemoryRegionSection *section,