[Qemu-devel] [PATCH v2 5/8] migration/ram: ensure write persistence on loading zero pages to PMEM

[Haozhong Zhang]

When loading a zero page, check whether it will be loaded to
persistent memory If yes, load it by libpmem function
pmem_memset_nodrain().  Combined with a call to pmem_drain() at the
end of RAM loading, we can guarantee all those zero pages are
persistently loaded.

Depending on the host HW/SW configurations, pmem_drain() can be
"sfence".  Therefore, we do not call pmem_drain() after each
pmem_memset_nodrain(), or use pmem_memset_persist() (equally
pmem_memset_nodrain() + pmem_drain()), in order to avoid unnecessary
overhead.

Signed-off-by: Haozhong Zhang <address@hidden>
---
 include/qemu/pmem.h |  9 +++++++++
 migration/ram.c     | 34 +++++++++++++++++++++++++++++-----
 2 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/include/qemu/pmem.h b/include/qemu/pmem.h
index 9017596ff0..861d8ecc21 100644
--- a/include/qemu/pmem.h
+++ b/include/qemu/pmem.h
@@ -26,6 +26,15 @@ pmem_memcpy_persist(void *pmemdest, const void *src, size_t 
len)
     return memcpy(pmemdest, src, len);
 }
 
+static inline void *pmem_memset_nodrain(void *pmemdest, int c, size_t len)
+{
+    return memset(pmemdest, c, len);
+}
+
+static inline void pmem_drain(void)
+{
+}
+
 #endif /* CONFIG_LIBPMEM */
 
 #endif /* !QEMU_PMEM_H */
diff --git a/migration/ram.c b/migration/ram.c
index cb1950f3eb..5a0e503818 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -49,6 +49,7 @@
 #include "qemu/rcu_queue.h"
 #include "migration/colo.h"
 #include "migration/block.h"
+#include "qemu/pmem.h"
 
 /***********************************************************/
 /* ram save/restore */
@@ -2467,6 +2468,20 @@ static inline void *host_from_ram_block_offset(RAMBlock 
*block,
     return block->host + offset;
 }
 
+static void ram_handle_compressed_common(void *host, uint8_t ch, uint64_t size,
+                                         bool is_pmem)
+{
+    if (!ch && is_zero_range(host, size)) {
+        return;
+    }
+
+    if (!is_pmem) {
+        memset(host, ch, size);
+    } else {
+        pmem_memset_nodrain(host, ch, size);
+    }
+}
+
 /**
  * ram_handle_compressed: handle the zero page case
  *
@@ -2479,9 +2494,7 @@ static inline void *host_from_ram_block_offset(RAMBlock 
*block,
  */
 void ram_handle_compressed(void *host, uint8_t ch, uint64_t size)
 {
-    if (ch != 0 || !is_zero_range(host, size)) {
-        memset(host, ch, size);
-    }
+    return ram_handle_compressed_common(host, ch, size, false);
 }
 
 static void *do_data_decompress(void *opaque)
@@ -2823,6 +2836,7 @@ static int ram_load(QEMUFile *f, void *opaque, int 
version_id)
     bool postcopy_running = postcopy_is_running();
     /* ADVISE is earlier, it shows the source has the postcopy capability on */
     bool postcopy_advised = postcopy_is_advised();
+    bool need_pmem_drain = false;
 
     seq_iter++;
 
@@ -2848,6 +2862,8 @@ static int ram_load(QEMUFile *f, void *opaque, int 
version_id)
         ram_addr_t addr, total_ram_bytes;
         void *host = NULL;
         uint8_t ch;
+        RAMBlock *block = NULL;
+        bool is_pmem = false;
 
         addr = qemu_get_be64(f);
         flags = addr & ~TARGET_PAGE_MASK;
@@ -2864,7 +2880,7 @@ static int ram_load(QEMUFile *f, void *opaque, int 
version_id)
 
         if (flags & (RAM_SAVE_FLAG_ZERO | RAM_SAVE_FLAG_PAGE |
                      RAM_SAVE_FLAG_COMPRESS_PAGE | RAM_SAVE_FLAG_XBZRLE)) {
-            RAMBlock *block = ram_block_from_stream(f, flags);
+            block = ram_block_from_stream(f, flags);
 
             host = host_from_ram_block_offset(block, addr);
             if (!host) {
@@ -2874,6 +2890,9 @@ static int ram_load(QEMUFile *f, void *opaque, int 
version_id)
             }
             ramblock_recv_bitmap_set(block, host);
             trace_ram_load_loop(block->idstr, (uint64_t)addr, flags, host);
+
+            is_pmem = ramblock_is_pmem(block);
+            need_pmem_drain = need_pmem_drain || is_pmem;
         }
 
         switch (flags & ~RAM_SAVE_FLAG_CONTINUE) {
@@ -2927,7 +2946,7 @@ static int ram_load(QEMUFile *f, void *opaque, int 
version_id)
 
         case RAM_SAVE_FLAG_ZERO:
             ch = qemu_get_byte(f);
-            ram_handle_compressed(host, ch, TARGET_PAGE_SIZE);
+            ram_handle_compressed_common(host, ch, TARGET_PAGE_SIZE, is_pmem);
             break;
 
         case RAM_SAVE_FLAG_PAGE:
@@ -2970,6 +2989,11 @@ static int ram_load(QEMUFile *f, void *opaque, int 
version_id)
     }
 
     wait_for_decompress_done();
+
+    if (need_pmem_drain) {
+        pmem_drain();
+    }
+
     rcu_read_unlock();
     trace_ram_load_complete(ret, seq_iter);
     return ret;
-- 
2.14.1