libvhost-user: Claim support for postcopy

Tell QEMU we understand the protocol features needed for postcopy.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

contrib/libvhost-user/libvhost-user.c

@@ -185,6 +185,35 @@ vmsg_close_fds(VhostUserMsg *vmsg)
     }
 }
 
+/* A test to see if we have userfault available */
+static bool
+have_userfault(void)
+{
+#if defined(__linux__) && defined(__NR_userfaultfd) &&\
+        defined(UFFD_FEATURE_MISSING_SHMEM) &&\
+        defined(UFFD_FEATURE_MISSING_HUGETLBFS)
+    /* Now test the kernel we're running on really has the features */
+    int ufd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+    struct uffdio_api api_struct;
+    if (ufd < 0) {
+        return false;
+    }
+
+    api_struct.api = UFFD_API;
+    api_struct.features = UFFD_FEATURE_MISSING_SHMEM |
+                          UFFD_FEATURE_MISSING_HUGETLBFS;
+    if (ioctl(ufd, UFFDIO_API, &api_struct)) {
+        close(ufd);
+        return false;
+    }
+    close(ufd);
+    return true;
+
+#else
+    return false;
+#endif
+}
+
 static bool
 vu_message_read(VuDev *dev, int conn_fd, VhostUserMsg *vmsg)
 {
@@ -939,6 +968,10 @@ vu_get_protocol_features_exec(VuDev *dev, VhostUserMsg *vmsg)
     uint64_t features = 1ULL << VHOST_USER_PROTOCOL_F_LOG_SHMFD |
                         1ULL << VHOST_USER_PROTOCOL_F_SLAVE_REQ;
 
+    if (have_userfault()) {
+        features |= 1ULL << VHOST_USER_PROTOCOL_F_PAGEFAULT;
+    }
+
     if (dev->iface->get_protocol_features) {
         features |= dev->iface->get_protocol_features(dev);
     }