powerpc/pseries: Move vas_migration_handler early during migration

Bugzilla: https://bugzilla.redhat.com/2130348

commit 465dda9
Author: Haren Myneni <haren@linux.ibm.com>
Date:   Thu Sep 22 01:27:07 2022 -0700

    powerpc/pseries: Move vas_migration_handler early during migration

    When the migration is initiated, the hypervisor changes VAS
    mappings as part of pre-migration event. Then the OS gets the
    migration event which closes all VAS windows before the migration
    starts. NX generates continuous faults until windows are closed
    and the user space can not differentiate these NX faults coming
    from the actual migration. So to reduce this time window, close
    VAS windows first in pseries_migrate_partition().

    Signed-off-by: Haren Myneni <haren@linux.ibm.com>
    Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/d8efade91dda831c9ed4abb226dab627da594c5f.camel@linux.ibm.com

Signed-off-by: Mukesh Chaurasiya <mchauras@redhat.com>

Author: Mukesh Chaurasiya

arch/powerpc/platforms/pseries/mobility.c

@@ -740,11 +740,19 @@ static int pseries_migrate_partition(u64 handle)
 #ifdef CONFIG_PPC_WATCHDOG
 	factor = nmi_wd_lpm_factor;
 #endif
+	/*
+	 * When the migration is initiated, the hypervisor changes VAS
+	 * mappings to prepare before OS gets the notification and
+	 * closes all VAS windows. NX generates continuous faults during
+	 * this time and the user space can not differentiate these
+	 * faults from the migration event. So reduce this time window
+	 * by closing VAS windows at the beginning of this function.
+	 */
+	vas_migration_handler(VAS_SUSPEND);
+
 	ret = wait_for_vasi_session_suspending(handle);
 	if (ret)
-		return ret;
-
-	vas_migration_handler(VAS_SUSPEND);
+		goto out;
 
 	if (factor)
 		watchdog_nmi_set_timeout_pct(factor);
@@ -765,6 +773,7 @@ static int pseries_migrate_partition(u64 handle)
 	if (factor)
 		watchdog_nmi_set_timeout_pct(0);
 
+out:
 	vas_migration_handler(VAS_RESUME);
 
 	return ret;