postcopy: use UFFDIO_ZEROPAGE only when available

Use a flag on the RAMBlock to state whether it has the
UFFDIO_ZEROPAGE capability, use it when it's available.

This allows the use of postcopy on tmpfs as well as hugepage
backed files.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

exec.c

@@ -99,6 +99,11 @@ static MemoryRegion io_mem_unassigned;
  */
 #define RAM_RESIZEABLE (1 << 2)
 
+/* UFFDIO_ZEROPAGE is available on this RAMBlock to atomically
+ * zero the page and wake waiting processes.
+ * (Set during postcopy)
+ */
+#define RAM_UF_ZEROPAGE (1 << 3)
 #endif
 
 #ifdef TARGET_PAGE_BITS_VARY
@@ -1767,6 +1772,17 @@ bool qemu_ram_is_shared(RAMBlock *rb)
     return rb->flags & RAM_SHARED;
 }
 
+/* Note: Only set at the start of postcopy */
+bool qemu_ram_is_uf_zeroable(RAMBlock *rb)
+{
+    return rb->flags & RAM_UF_ZEROPAGE;
+}
+
+void qemu_ram_set_uf_zeroable(RAMBlock *rb)
+{
+    rb->flags |= RAM_UF_ZEROPAGE;
+}
+
 /* Called with iothread lock held.  */
 void qemu_ram_set_idstr(RAMBlock *new_block, const char *name, DeviceState *dev)
 {

include/exec/cpu-common.h

@@ -73,6 +73,9 @@ void qemu_ram_set_idstr(RAMBlock *block, const char *name, DeviceState *dev);
 void qemu_ram_unset_idstr(RAMBlock *block);
 const char *qemu_ram_get_idstr(RAMBlock *rb);
 bool qemu_ram_is_shared(RAMBlock *rb);
+bool qemu_ram_is_uf_zeroable(RAMBlock *rb);
+void qemu_ram_set_uf_zeroable(RAMBlock *rb);
+
 size_t qemu_ram_pagesize(RAMBlock *block);
 size_t qemu_ram_pagesize_largest(void);
 

migration/postcopy-ram.c

@@ -481,6 +481,10 @@ static int ram_block_enable_notify(const char *block_name, void *host_addr,
         error_report("%s userfault: Region doesn't support COPY", __func__);
         return -1;
     }
+    if (reg_struct.ioctls & ((__u64)1 << _UFFDIO_ZEROPAGE)) {
+        RAMBlock *rb = qemu_ram_block_by_name(block_name);
+        qemu_ram_set_uf_zeroable(rb);
+    }
 
     return 0;
 }
@@ -700,11 +704,14 @@ int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from,
 int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
                              RAMBlock *rb)
 {
+    size_t pagesize = qemu_ram_pagesize(rb);
     trace_postcopy_place_page_zero(host);
 
-    if (qemu_ram_pagesize(rb) == getpagesize()) {
-        if (qemu_ufd_copy_ioctl(mis->userfault_fd, host, NULL, getpagesize(),
-                                rb)) {
+    /* Normal RAMBlocks can zero a page using UFFDIO_ZEROPAGE
+     * but it's not available for everything (e.g. hugetlbpages)
+     */
+    if (qemu_ram_is_uf_zeroable(rb)) {
+        if (qemu_ufd_copy_ioctl(mis->userfault_fd, host, NULL, pagesize, rb)) {
             int e = errno;
             error_report("%s: %s zero host: %p",
                          __func__, strerror(e), host);