@@ -3827,6 +3827,12 @@ static void *bg_migration_thread(void *opaque)
update_iteration_initial_status(s);
+ /*
+ * Prepare for tracking memory writes with UFFD-WP - populate
+ * RAM pages before protecting.
+ */
+ ram_write_tracking_prepare();
+
qemu_savevm_state_header(s->to_dst_file);
qemu_savevm_state_setup(s->to_dst_file);
@@ -1560,6 +1560,54 @@ out:
return ret;
}
+/*
+ * ram_block_populate_pages: populate memory in the RAM block by reading
+ * an integer from the beginning of each page.
+ *
+ * Since it's solely used for userfault_fd WP feature, here we just
+ * hardcode page size to qemu_real_host_page_size.
+ *
+ * @bs: RAM block to populate
+ */
+static void ram_block_populate_pages(RAMBlock *bs)
+{
+ char *ptr = (char *) bs->host;
+
+ for (ram_addr_t offset = 0; offset < bs->used_length;
+ offset += qemu_real_host_page_size) {
+ char tmp = *(ptr + offset);
+ /* Don't optimize the read out */
+ asm volatile("" : "+r" (tmp));
+ }
+}
+
+/*
+ * ram_write_tracking_prepare: prepare for UFFD-WP memory tracking
+ */
+void ram_write_tracking_prepare(void)
+{
+ RAMBlock *bs;
+
+ RCU_READ_LOCK_GUARD();
+
+ RAMBLOCK_FOREACH_NOT_IGNORED(bs) {
+ /* Nothing to do with read-only and MMIO-writable regions */
+ if (bs->mr->readonly || bs->mr->rom_device) {
+ continue;
+ }
+
+ /*
+ * Populate pages of the RAM block before enabling userfault_fd
+ * write protection.
+ *
+ * This stage is required since ioctl(UFFDIO_WRITEPROTECT) with
+ * UFFDIO_WRITEPROTECT_MODE_WP mode setting would silently skip
+ * pages with pte_none() entries in page table.
+ */
+ ram_block_populate_pages(bs);
+ }
+}
+
/*
* ram_write_tracking_start: start UFFD-WP memory tracking
*
@@ -82,6 +82,7 @@ void colo_incoming_start_dirty_log(void);
/* Background snapshot */
bool ram_write_tracking_available(void);
bool ram_write_tracking_compatible(void);
+void ram_write_tracking_prepare(void);
int ram_write_tracking_start(void);
void ram_write_tracking_stop(void);
This commit solves the issue with userfault_fd WP feature that background snapshot is based on. For any never poluated or discarded memory page, the UFFDIO_WRITEPROTECT ioctl() would skip updating PTE for that page, thereby loosing WP setting for it. So we need to pre-fault pages for each RAM block to be protected before making a userfault_fd wr-protect ioctl(). Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com> --- migration/migration.c | 6 ++++++ migration/ram.c | 48 +++++++++++++++++++++++++++++++++++++++++++ migration/ram.h | 1 + 3 files changed, 55 insertions(+)