1 files changed, 88 insertions, 6 deletions

diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index cf62fc756f..0de68e8b25 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -61,16 +61,67 @@ struct PostcopyDiscardState {
 #include <sys/eventfd.h>
 #include <linux/userfaultfd.h>
 
-static bool ufd_version_check(int ufd, MigrationIncomingState *mis)
+
+/**
+ * receive_ufd_features: check userfault fd features, to request only supported
+ * features in the future.
+ *
+ * Returns: true on success
+ *
+ * __NR_userfaultfd - should be checked before
+ *  @features: out parameter will contain uffdio_api.features provided by kernel
+ *              in case of success
+ */
+static bool receive_ufd_features(uint64_t *features)
 {
-    struct uffdio_api api_struct;
-    uint64_t ioctl_mask;
+    struct uffdio_api api_struct = {0};
+    int ufd;
+    bool ret = true;
+
+    /* if we are here __NR_userfaultfd should exists */
+    ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
+    if (ufd == -1) {
+        error_report("%s: syscall __NR_userfaultfd failed: %s", __func__,
+                     strerror(errno));
+        return false;
+    }
 
+    /* ask features */
     api_struct.api = UFFD_API;
     api_struct.features = 0;
     if (ioctl(ufd, UFFDIO_API, &api_struct)) {
         error_report("%s: UFFDIO_API failed: %s", __func__,
                      strerror(errno));
+        ret = false;
+        goto release_ufd;
+    }
+
+    *features = api_struct.features;
+
+release_ufd:
+    close(ufd);
+    return ret;
+}
+
+/**
+ * request_ufd_features: this function should be called only once on a newly
+ * opened ufd, subsequent calls will lead to error.
+ *
+ * Returns: true on succes
+ *
+ * @ufd: fd obtained from userfaultfd syscall
+ * @features: bit mask see UFFD_API_FEATURES
+ */
+static bool request_ufd_features(int ufd, uint64_t features)
+{
+    struct uffdio_api api_struct = {0};
+    uint64_t ioctl_mask;
+
+    api_struct.api = UFFD_API;
+    api_struct.features = features;
+    if (ioctl(ufd, UFFDIO_API, &api_struct)) {
+        error_report("%s failed: UFFDIO_API failed: %s", __func__,
+                     strerror(errno));
         return false;
     }
 
@@ -82,11 +133,42 @@ static bool ufd_version_check(int ufd, MigrationIncomingState *mis)
         return false;
     }
 
+    return true;
+}
+
+static bool ufd_check_and_apply(int ufd, MigrationIncomingState *mis)
+{
+    uint64_t asked_features = 0;
+    static uint64_t supported_features;
+
+    /*
+     * it's not possible to
+     * request UFFD_API twice per one fd
+     * userfault fd features is persistent
+     */
+    if (!supported_features) {
+        if (!receive_ufd_features(&supported_features)) {
+            error_report("%s failed", __func__);
+            return false;
+        }
+    }
+
+    /*
+     * request features, even if asked_features is 0, due to
+     * kernel expects UFFD_API before UFFDIO_REGISTER, per
+     * userfault file descriptor
+     */
+    if (!request_ufd_features(ufd, asked_features)) {
+        error_report("%s failed: features %" PRIu64, __func__,
+                     asked_features);
+        return false;
+    }
+
     if (getpagesize() != ram_pagesize_summary()) {
         bool have_hp = false;
         /* We've got a huge page */
 #ifdef UFFD_FEATURE_MISSING_HUGETLBFS
-        have_hp = api_struct.features & UFFD_FEATURE_MISSING_HUGETLBFS;
+        have_hp = supported_features & UFFD_FEATURE_MISSING_HUGETLBFS;
 #endif
         if (!have_hp) {
             error_report("Userfault on this host does not support huge pages");
@@ -147,7 +229,7 @@ bool postcopy_ram_supported_by_host(MigrationIncomingState *mis)
     }
 
     /* Version and features check */
-    if (!ufd_version_check(ufd, mis)) {
+    if (!ufd_check_and_apply(ufd, mis)) {
         goto out;
     }
 
@@ -523,7 +605,7 @@ int postcopy_ram_enable_notify(MigrationIncomingState *mis)
      * Although the host check already tested the API, we need to
      * do the check again as an ABI handshake on the new fd.
      */
-    if (!ufd_version_check(mis->userfault_fd, mis)) {
+    if (!ufd_check_and_apply(mis->userfault_fd, mis)) {
         return -1;
     }