[Qemu-devel] [RFC 5/6] memory: replace cpu_physical_memory_reset_dirty() with test-and-clear

[Stefan Hajnoczi]

The cpu_physical_memory_reset_dirty() function is sometimes used
together with cpu_physical_memory_get_dirty().  This is not atomic since
two separate accesses to the dirty memory bitmap are made.

Turn cpu_physical_memory_reset_dirty() into the atomic
cpu_physical_memory_test_and_clear_dirty().

The cpu_physical_memory_clear_dirty_range() function is no longer used
and can be dropped.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 cputlb.c                |  4 ++--
 exec.c                  | 23 +++++++++++++++++------
 include/exec/ram_addr.h | 27 +++++++--------------------
 memory.c                | 11 ++++-------
 4 files changed, 30 insertions(+), 35 deletions(-)

diff --git a/cputlb.c b/cputlb.c
index a55518a..3d5b9a7 100644
--- a/cputlb.c
+++ b/cputlb.c
@@ -125,8 +125,8 @@ void tlb_flush_page(CPUState *cpu, target_ulong addr)
    can be detected */
 void tlb_protect_code(ram_addr_t ram_addr)
 {
-    cpu_physical_memory_reset_dirty(ram_addr, TARGET_PAGE_SIZE,
-                                    DIRTY_MEMORY_CODE);
+    cpu_physical_memory_test_and_clear_dirty(ram_addr, TARGET_PAGE_SIZE,
+                                             DIRTY_MEMORY_CODE);
 }
 
 /* update the TLB so that writes in physical page 'phys_addr' are no longer
diff --git a/exec.c b/exec.c
index 71ac104..72bdb2e 100644
--- a/exec.c
+++ b/exec.c
@@ -845,16 +845,27 @@ static void tlb_reset_dirty_range_all(ram_addr_t start, 
ram_addr_t length)
 }
 
 /* Note: start and end must be within the same ram block.  */
-void cpu_physical_memory_reset_dirty(ram_addr_t start, ram_addr_t length,
-                                     unsigned client)
+bool cpu_physical_memory_test_and_clear_dirty(ram_addr_t start,
+                                              ram_addr_t length,
+                                              unsigned client)
 {
-    if (length == 0)
-        return;
-    cpu_physical_memory_clear_dirty_range(start, length, client);
+    unsigned long end, page;
+    bool dirty;
 
-    if (tcg_enabled()) {
+    if (length == 0) {
+        return false;
+    }
+
+    end = TARGET_PAGE_ALIGN(start + length) >> TARGET_PAGE_BITS;
+    page = start >> TARGET_PAGE_BITS;
+    dirty = bitmap_test_and_clear_atomic(ram_list.dirty_memory[client],
+                                         page, end - page);
+
+    if (dirty && tcg_enabled()) {
         tlb_reset_dirty_range_all(start, length);
     }
+
+    return dirty;
 }
 
 static void cpu_physical_memory_set_dirty_tracking(bool enable)
diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index 87a8b28..cdcbe9a 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -175,20 +175,9 @@ static inline void 
cpu_physical_memory_set_dirty_lebitmap(unsigned long *bitmap,
 }
 #endif /* not _WIN32 */
 
-static inline void cpu_physical_memory_clear_dirty_range(ram_addr_t start,
-                                                         ram_addr_t length,
-                                                         unsigned client)
-{
-    unsigned long end, page;
-
-    assert(client < DIRTY_MEMORY_NUM);
-    end = TARGET_PAGE_ALIGN(start + length) >> TARGET_PAGE_BITS;
-    page = start >> TARGET_PAGE_BITS;
-    bitmap_clear(ram_list.dirty_memory[client], page, end - page);
-}
-
-void cpu_physical_memory_reset_dirty(ram_addr_t start, ram_addr_t length,
-                                     unsigned client);
+bool cpu_physical_memory_test_and_clear_dirty(ram_addr_t start,
+                                              ram_addr_t length,
+                                              unsigned client);
 
 static inline
 uint64_t cpu_physical_memory_sync_dirty_bitmap(unsigned long *dest,
@@ -217,12 +206,10 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(unsigned 
long *dest,
         }
     } else {
         for (addr = 0; addr < length; addr += TARGET_PAGE_SIZE) {
-            if (cpu_physical_memory_get_dirty(start + addr,
-                                              TARGET_PAGE_SIZE,
-                                              DIRTY_MEMORY_MIGRATION)) {
-                cpu_physical_memory_reset_dirty(start + addr,
-                                                TARGET_PAGE_SIZE,
-                                                DIRTY_MEMORY_MIGRATION);
+            if (cpu_physical_memory_test_and_clear_dirty(
+                        start + addr,
+                        TARGET_PAGE_SIZE,
+                        DIRTY_MEMORY_MIGRATION)) {
                 long k = (start + addr) >> TARGET_PAGE_BITS;
                 if (!test_and_set_bit(k, dest)) {
                     num_dirty++;
diff --git a/memory.c b/memory.c
index 15cf9eb..4d7761e 100644
--- a/memory.c
+++ b/memory.c
@@ -1375,13 +1375,9 @@ void memory_region_set_dirty(MemoryRegion *mr, hwaddr 
addr,
 bool memory_region_test_and_clear_dirty(MemoryRegion *mr, hwaddr addr,
                                         hwaddr size, unsigned client)
 {
-    bool ret;
     assert(mr->terminates);
-    ret = cpu_physical_memory_get_dirty(mr->ram_addr + addr, size, client);
-    if (ret) {
-        cpu_physical_memory_reset_dirty(mr->ram_addr + addr, size, client);
-    }
-    return ret;
+    return cpu_physical_memory_test_and_clear_dirty(mr->ram_addr + addr,
+                                                    size, client);
 }
 
 
@@ -1425,7 +1421,8 @@ void memory_region_reset_dirty(MemoryRegion *mr, hwaddr 
addr,
                                hwaddr size, unsigned client)
 {
     assert(mr->terminates);
-    cpu_physical_memory_reset_dirty(mr->ram_addr + addr, size, client);
+    cpu_physical_memory_test_and_clear_dirty(mr->ram_addr + addr, size,
+                                             client);
 }
 
 int memory_region_get_fd(MemoryRegion *mr)
-- 
2.1.0