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 <armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

1 files changed, 35 insertions, 0 deletions

diff --git a/migration/options.c b/migration/options.c
index 5ab5b6d85d..645f55003d 100644
--- a/migration/options.c
+++ b/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)