[libvirt PATCH] qemu: Enable unprivileged userfaultfd for post-copy migration

[Jiri Denemark]

Userfaultfd is by default allowed only for privileged processes. Since
libvirt runs QEMU unprivileged, we need to enable unprivileged access to
userfaultfd before starting post-copy migration.

Rather than providing a static sysctl configuration file, we set the
sysctl knob in runtime once post-copy migration is requested. This way
unprivileged_userfaultfd is only enabled once actually used.

https://bugzilla.redhat.com/show_bug.cgi?id=1945420

Signed-off-by: Jiri Denemark <jdenemar at redhat.com>
---
 src/qemu/qemu_migration_params.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/src/qemu/qemu_migration_params.c b/src/qemu/qemu_migration_params.c
index dbc3219826..a9449ed1ff 100644
--- a/src/qemu/qemu_migration_params.c
+++ b/src/qemu/qemu_migration_params.c
@@ -804,6 +804,24 @@ qemuMigrationCapsToJSON(virBitmap *caps,
 }
 
 
+static void
+qemuMigrationParamsEnableUserfaultfd(virDomainObj *vm)
+{
+    const char *sysctl = "/proc/sys/vm/unprivileged_userfaultfd";
+
+    if (!virFileExists(sysctl))
+        return;
+
+    VIR_DEBUG("Enabling unprivileged userfaultfd for post-copy migration of "
+              "domain %s", vm->def->name);
+
+    if (virFileWriteStr(sysctl, "1", 0) < 0) {
+        virReportSystemError(errno, "%s",
+                             _("failed to enable unprivileged userfaultfd"));
+    }
+}
+
+
 /**
  * qemuMigrationParamsApply
  * @driver: qemu driver
@@ -839,6 +857,13 @@ qemuMigrationParamsApply(virQEMUDriver *driver,
             goto cleanup;
         }
     } else {
+        /* userfaultfd may only be enabled for privileged processes by default,
+         * we need to make sure QEMU can use it before enabling post-copy
+         * migration */
+        if (virBitmapIsBitSet(priv->migrationCaps, QEMU_MIGRATION_CAP_POSTCOPY) &&
+            virBitmapIsBitSet(migParams->caps, QEMU_MIGRATION_CAP_POSTCOPY))
+            qemuMigrationParamsEnableUserfaultfd(vm);
+
         if (!(caps = qemuMigrationCapsToJSON(priv->migrationCaps, migParams->caps)))
             goto cleanup;
 
-- 
2.34.1