[PATCH v2 28/29] migration: Add direct-io parameter

[Fabiano Rosas]

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 for the secondary channels of fixed-ram
migration, which can guarantee that writes are page aligned.

However the parameter could be made to affect other types of
file-based migrations in the future.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 include/qemu/osdep.h           |  2 ++
 migration/file.c               | 15 ++++++++++++---
 migration/migration-hmp-cmds.c | 10 ++++++++++
 migration/options.c            | 30 ++++++++++++++++++++++++++++++
 migration/options.h            |  1 +
 qapi/migration.json            | 17 ++++++++++++++---
 util/osdep.c                   |  9 +++++++++
 7 files changed, 78 insertions(+), 6 deletions(-)

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 475a1c62ff..ea5d29ab9b 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -597,6 +597,8 @@ 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)
diff --git a/migration/file.c b/migration/file.c
index ad75225f43..3d3c58ecad 100644
--- a/migration/file.c
+++ b/migration/file.c
@@ -11,9 +11,9 @@
 #include "qemu/error-report.h"
 #include "channel.h"
 #include "file.h"
-#include "migration.h"
 #include "io/channel-file.h"
 #include "io/channel-util.h"
+#include "migration.h"
 #include "options.h"
 #include "trace.h"
 
@@ -77,9 +77,18 @@ void file_send_channel_create(QIOTaskFunc f, void *data)
     QIOChannelFile *ioc;
     QIOTask *task;
     Error *errp = NULL;
+    int flags = outgoing_args.flags;
 
-    ioc = qio_channel_file_new_path(outgoing_args.fname,
-                                    outgoing_args.flags,
+    if (migrate_direct_io() && qemu_has_direct_io()) {
+        /*
+         * Enable O_DIRECT for the secondary channels. These are used
+         * for sending ram pages and writes should be guaranteed to be
+         * aligned to at least page size.
+         */
+        flags |= O_DIRECT;
+    }
+
+    ioc = qio_channel_file_new_path(outgoing_args.fname, flags,
                                     outgoing_args.mode, &errp);
     if (!ioc) {
         file_migration_cancel(errp);
diff --git a/migration/migration-hmp-cmds.c b/migration/migration-hmp-cmds.c
index a82597f18e..eab5ac3588 100644
--- a/migration/migration-hmp-cmds.c
+++ b/migration/migration-hmp-cmds.c
@@ -387,6 +387,12 @@ void hmp_info_migrate_parameters(Monitor *mon, const QDict 
*qdict)
         monitor_printf(mon, "%s: %" PRIu64 " MB/s\n",
             MigrationParameter_str(MIGRATION_PARAMETER_VCPU_DIRTY_LIMIT),
             params->vcpu_dirty_limit);
+
+        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);
@@ -661,6 +667,10 @@ void hmp_migrate_set_parameter(Monitor *mon, const QDict 
*qdict)
         p->has_vcpu_dirty_limit = true;
         visit_type_size(v, param, &p->vcpu_dirty_limit, &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);
     }
diff --git a/migration/options.c b/migration/options.c
index 2193d69e71..6d0e3c26ae 100644
--- a/migration/options.c
+++ b/migration/options.c
@@ -817,6 +817,22 @@ int migrate_decompress_threads(void)
     return s->parameters.decompress_threads;
 }
 
+bool migrate_direct_io(void)
+{
+    MigrationState *s = migrate_get_current();
+
+    /* For now O_DIRECT is only supported with fixed-ram */
+    if (!s->capabilities[MIGRATION_CAPABILITY_FIXED_RAM]) {
+        return false;
+    }
+
+    if (s->parameters.has_direct_io) {
+        return s->parameters.direct_io;
+    }
+
+    return false;
+}
+
 uint64_t migrate_downtime_limit(void)
 {
     MigrationState *s = migrate_get_current();
@@ -1025,6 +1041,11 @@ MigrationParameters *qmp_query_migrate_parameters(Error 
**errp)
     params->has_vcpu_dirty_limit = true;
     params->vcpu_dirty_limit = s->parameters.vcpu_dirty_limit;
 
+    if (s->parameters.has_direct_io) {
+        params->has_direct_io = true;
+        params->direct_io = s->parameters.direct_io;
+    }
+
     return params;
 }
 
@@ -1059,6 +1080,7 @@ void migrate_params_init(MigrationParameters *params)
     params->has_announce_step = true;
     params->has_x_vcpu_dirty_limit_period = true;
     params->has_vcpu_dirty_limit = true;
+    params->has_direct_io = qemu_has_direct_io();
 }
 
 /*
@@ -1356,6 +1378,10 @@ static void 
migrate_params_test_apply(MigrateSetParameters *params,
     if (params->has_vcpu_dirty_limit) {
         dest->vcpu_dirty_limit = params->vcpu_dirty_limit;
     }
+
+    if (params->has_direct_io) {
+        dest->direct_io = params->direct_io;
+    }
 }
 
 static void migrate_params_apply(MigrateSetParameters *params, Error **errp)
@@ -1486,6 +1512,10 @@ static void migrate_params_apply(MigrateSetParameters 
*params, Error **errp)
     if (params->has_vcpu_dirty_limit) {
         s->parameters.vcpu_dirty_limit = params->vcpu_dirty_limit;
     }
+
+    if (params->has_direct_io) {
+        s->parameters.direct_io = params->direct_io;
+    }
 }
 
 void qmp_migrate_set_parameters(MigrateSetParameters *params, Error **errp)
diff --git a/migration/options.h b/migration/options.h
index 01bba5b928..280f86bed1 100644
--- a/migration/options.h
+++ b/migration/options.h
@@ -82,6 +82,7 @@ uint8_t migrate_cpu_throttle_increment(void);
 uint8_t migrate_cpu_throttle_initial(void);
 bool migrate_cpu_throttle_tailslow(void);
 int migrate_decompress_threads(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);
diff --git a/qapi/migration.json b/qapi/migration.json
index 1317dd32ab..3eb9e2c9b5 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -840,6 +840,9 @@
 # @vcpu-dirty-limit: Dirtyrate limit (MB/s) during live migration.
 #     Defaults to 1.  (Since 8.1)
 #
+# @direct-io: Open migration files with O_DIRECT when possible. Not
+#             all migration transports support this. (since 8.1)
+#
 # Features:
 #
 # @unstable: Members @x-checkpoint-delay and @x-vcpu-dirty-limit-period
@@ -864,7 +867,7 @@
            'multifd-zlib-level', 'multifd-zstd-level',
            'block-bitmap-mapping',
            { 'name': 'x-vcpu-dirty-limit-period', 'features': ['unstable'] },
-           'vcpu-dirty-limit'] }
+           'vcpu-dirty-limit', 'direct-io'] }
 
 ##
 # @MigrateSetParameters:
@@ -1016,6 +1019,9 @@
 # @vcpu-dirty-limit: Dirtyrate limit (MB/s) during live migration.
 #     Defaults to 1.  (Since 8.1)
 #
+# @direct-io: Open migration files with O_DIRECT when possible. Not
+#             all migration transports support this. (since 8.1)
+#
 # Features:
 #
 # @unstable: Members @x-checkpoint-delay and @x-vcpu-dirty-limit-period
@@ -1058,7 +1064,8 @@
             '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ],
             '*x-vcpu-dirty-limit-period': { 'type': 'uint64',
                                             'features': [ 'unstable' ] },
-            '*vcpu-dirty-limit': 'uint64'} }
+            '*vcpu-dirty-limit': 'uint64',
+            '*direct-io': 'bool' } }
 
 ##
 # @migrate-set-parameters:
@@ -1230,6 +1237,9 @@
 # @vcpu-dirty-limit: Dirtyrate limit (MB/s) during live migration.
 #     Defaults to 1.  (Since 8.1)
 #
+# @direct-io: Open migration files with O_DIRECT when possible. Not
+#             all migration transports support this. (since 8.1)
+#
 # Features:
 #
 # @unstable: Members @x-checkpoint-delay and @x-vcpu-dirty-limit-period
@@ -1269,7 +1279,8 @@
             '*block-bitmap-mapping': [ 'BitmapMigrationNodeAlias' ],
             '*x-vcpu-dirty-limit-period': { 'type': 'uint64',
                                             'features': [ 'unstable' ] },
-            '*vcpu-dirty-limit': 'uint64'} }
+            '*vcpu-dirty-limit': 'uint64',
+            '*direct-io': 'bool' } }
 
 ##
 # @query-migrate-parameters:
diff --git a/util/osdep.c b/util/osdep.c
index e996c4744a..d0227a60ab 100644
--- a/util/osdep.c
+++ b/util/osdep.c
@@ -277,6 +277,15 @@ 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;
-- 
2.35.3