migration: Add direct-io parameter

Add the direct-io migration parameter that tells the migration code to
use O_DIRECT when opening the migration stream file whenever possible.

This is currently only used with the mapped-ram migration that has a
clear window guaranteed to perform aligned writes.

Acked-by: Markus Armbruster <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Signed-off-by: Fabiano Rosas <[email protected]>

Author: Fabiano Rosas

include/qemu/osdep.h

@@ -612,6 +612,8 @@ int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive);
 bool qemu_has_ofd_lock(void);
 #endif
 
+bool qemu_has_direct_io(void);
+
 #if defined(__HAIKU__) && defined(__i386__)
 #define FMT_pid "%ld"
 #elif defined(WIN64)

migration/migration-hmp-cmds.c

@@ -351,6 +351,13 @@ void hmp_info_migrate_parameters(Monitor *mon, const QDict *qdict)
         monitor_printf(mon, "%s: %s\n",
             MigrationParameter_str(MIGRATION_PARAMETER_MODE),
             qapi_enum_lookup(&MigMode_lookup, params->mode));
+
+        if (params->has_direct_io) {
+            monitor_printf(mon, "%s: %s\n",
+                           MigrationParameter_str(
+                               MIGRATION_PARAMETER_DIRECT_IO),
+                           params->direct_io ? "on" : "off");
+        }
     }
 
     qapi_free_MigrationParameters(params);
@@ -624,6 +631,10 @@ void hmp_migrate_set_parameter(Monitor *mon, const QDict *qdict)
         p->has_mode = true;
         visit_type_MigMode(v, param, &p->mode, &err);
         break;
+    case MIGRATION_PARAMETER_DIRECT_IO:
+        p->has_direct_io = true;
+        visit_type_bool(v, param, &p->direct_io, &err);
+        break;
     default:
         assert(0);
     }

migration/options.c

@@ -702,6 +702,25 @@ bool migrate_cpu_throttle_tailslow(void)
     return s->parameters.cpu_throttle_tailslow;
 }
 
+bool migrate_direct_io(void)
+{
+    MigrationState *s = migrate_get_current();
+
+    /*
+     * O_DIRECT is only supported with mapped-ram and multifd.
+     *
+     * mapped-ram is needed because filesystems impose restrictions on
+     * O_DIRECT IO alignment (see MAPPED_RAM_FILE_OFFSET_ALIGNMENT).
+     *
+     * multifd is needed to keep the unaligned portion of the stream
+     * isolated to the main migration thread while multifd channels
+     * process the aligned data with O_DIRECT enabled.
+     */
+    return s->parameters.direct_io &&
+        s->capabilities[MIGRATION_CAPABILITY_MAPPED_RAM] &&
+        s->capabilities[MIGRATION_CAPABILITY_MULTIFD];
+}
+
 uint64_t migrate_downtime_limit(void)
 {
     MigrationState *s = migrate_get_current();
@@ -905,6 +924,8 @@ MigrationParameters *qmp_query_migrate_parameters(Error **errp)
     params->mode = s->parameters.mode;
     params->has_zero_page_detection = true;
     params->zero_page_detection = s->parameters.zero_page_detection;
+    params->has_direct_io = true;
+    params->direct_io = s->parameters.direct_io;
 
     return params;
 }
@@ -937,6 +958,7 @@ void migrate_params_init(MigrationParameters *params)
     params->has_vcpu_dirty_limit = true;
     params->has_mode = true;
     params->has_zero_page_detection = true;
+    params->has_direct_io = true;
 }
 
 /*
@@ -1110,6 +1132,11 @@ bool migrate_params_check(MigrationParameters *params, Error **errp)
         return false;
     }
 
+    if (params->has_direct_io && params->direct_io && !qemu_has_direct_io()) {
+        error_setg(errp, "No build-time support for direct-io");
+        return false;
+    }
+
     return true;
 }
 
@@ -1216,6 +1243,10 @@ static void migrate_params_test_apply(MigrateSetParameters *params,
     if (params->has_zero_page_detection) {
         dest->zero_page_detection = params->zero_page_detection;
     }
+
+    if (params->has_direct_io) {
+        dest->direct_io = params->direct_io;
+    }
 }
 
 static void migrate_params_apply(MigrateSetParameters *params, Error **errp)
@@ -1341,6 +1372,10 @@ static void migrate_params_apply(MigrateSetParameters *params, Error **errp)
     if (params->has_zero_page_detection) {
         s->parameters.zero_page_detection = params->zero_page_detection;
     }
+
+    if (params->has_direct_io) {
+        s->parameters.direct_io = params->direct_io;
+    }
 }
 
 void qmp_migrate_set_parameters(MigrateSetParameters *params, Error **errp)

migration/options.h

@@ -69,6 +69,7 @@ uint32_t migrate_checkpoint_delay(void);
 uint8_t migrate_cpu_throttle_increment(void);
 uint8_t migrate_cpu_throttle_initial(void);
 bool migrate_cpu_throttle_tailslow(void);
+bool migrate_direct_io(void);
 uint64_t migrate_downtime_limit(void);
 uint8_t migrate_max_cpu_throttle(void);
 uint64_t migrate_max_bandwidth(void);

qapi/migration.json

@@ -821,6 +821,10 @@
 #     See description in @ZeroPageDetection.  Default is 'multifd'.
 #     (since 9.0)
 #
+# @direct-io: Open migration files with O_DIRECT when possible.  This
+#     only has effect if the @mapped-ram capability is enabled.
+#     (Since 9.1)
+#
 # Features:
 #
 # @unstable: Members @x-checkpoint-delay and
@@ -845,7 +849,8 @@
            { 'name': 'x-vcpu-dirty-limit-period', 'features': ['unstable'] },
            'vcpu-dirty-limit',
            'mode',
-           'zero-page-detection'] }
+           'zero-page-detection',
+           'direct-io'] }
 
 ##
 # @MigrateSetParameters:
@@ -991,6 +996,10 @@
 #     See description in @ZeroPageDetection.  Default is 'multifd'.
 #     (since 9.0)
 #
+# @direct-io: Open migration files with O_DIRECT when possible.  This
+#     only has effect if the @mapped-ram capability is enabled.
+#     (Since 9.1)
+#
 # Features:
 #
 # @unstable: Members @x-checkpoint-delay and
@@ -1030,7 +1039,8 @@
                                             'features': [ 'unstable' ] },
             '*vcpu-dirty-limit': 'uint64',
             '*mode': 'MigMode',
-            '*zero-page-detection': 'ZeroPageDetection'} }
+            '*zero-page-detection': 'ZeroPageDetection',
+            '*direct-io': 'bool' } }
 
 ##
 # @migrate-set-parameters:
@@ -1190,6 +1200,10 @@
 #     See description in @ZeroPageDetection.  Default is 'multifd'.
 #     (since 9.0)
 #
+# @direct-io: Open migration files with O_DIRECT when possible.  This
+#     only has effect if the @mapped-ram capability is enabled.
+#     (Since 9.1)
+#
 # Features:
 #
 # @unstable: Members @x-checkpoint-delay and
@@ -1226,7 +1240,8 @@
                                             'features': [ 'unstable' ] },
             '*vcpu-dirty-limit': 'uint64',
             '*mode': 'MigMode',
-            '*zero-page-detection': 'ZeroPageDetection'} }
+            '*zero-page-detection': 'ZeroPageDetection',
+            '*direct-io': 'bool' } }
 
 ##
 # @query-migrate-parameters:

util/osdep.c

@@ -282,6 +282,15 @@ int qemu_lock_fd_test(int fd, int64_t start, int64_t len, bool exclusive)
 }
 #endif
 
+bool qemu_has_direct_io(void)
+{
+#ifdef O_DIRECT
+    return true;
+#else
+    return false;
+#endif
+}
+
 static int qemu_open_cloexec(const char *name, int flags, mode_t mode)
 {
     int ret;