migration: Add property x-postcopy-preempt-break-huge

Add a property field that can conditionally disable the "break sending huge
page" behavior in postcopy preemption.  By default it's enabled.

It should only be used for debugging purposes, and we should never remove
the "x-" prefix.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Manish Mishra <manish.mishra@nutanix.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220707185511.27366-1-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/migration.c

@@ -4363,6 +4363,8 @@ static Property migration_properties[] = {
     DEFINE_PROP_SIZE("announce-step", MigrationState,
                       parameters.announce_step,
                       DEFAULT_MIGRATE_ANNOUNCE_STEP),
+    DEFINE_PROP_BOOL("x-postcopy-preempt-break-huge", MigrationState,
+                      postcopy_preempt_break_huge, true),
 
     /* Migration capabilities */
     DEFINE_PROP_MIG_CAP("x-xbzrle", MIGRATION_CAPABILITY_XBZRLE),

migration/migration.h

@@ -340,6 +340,13 @@ struct MigrationState {
     bool send_configuration;
     /* Whether we send section footer during migration */
     bool send_section_footer;
+    /*
+     * Whether we allow break sending huge pages when postcopy preempt is
+     * enabled.  When disabled, we won't interrupt precopy within sending a
+     * host huge page, which is the old behavior of vanilla postcopy.
+     * NOTE: this parameter is ignored if postcopy preempt is not enabled.
+     */
+    bool postcopy_preempt_break_huge;
 
     /* Needed by postcopy-pause state */
     QemuSemaphore postcopy_pause_sem;

migration/ram.c

@@ -2266,11 +2266,18 @@ static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss)
 
 static bool postcopy_needs_preempt(RAMState *rs, PageSearchStatus *pss)
 {
+    MigrationState *ms = migrate_get_current();
+
     /* Not enabled eager preempt?  Then never do that. */
     if (!migrate_postcopy_preempt()) {
         return false;
     }
 
+    /* If the user explicitly disabled breaking of huge page, skip */
+    if (!ms->postcopy_preempt_break_huge) {
+        return false;
+    }
+
     /* If the ramblock we're sending is a small page?  Never bother. */
     if (qemu_ram_pagesize(pss->block) == TARGET_PAGE_SIZE) {
         return false;