Add a multifd test for fixed-ram with passing of fds into QEMU. This
is how libvirt will consume the feature.

There are a couple of details to the fdset mechanism:

- multifd needs two distinct file descriptors (not duplicated with
  dup()) on the outgoing side so it can enable O_DIRECT only on the
  channels that write with alignment. The dup() system call creates
  file descriptors that share status flags, of which O_DIRECT is one.

  the incoming side doesn't set O_DIRECT, so it can dup() fds and
  therefore can receive only one in the fdset.

- the open() access mode flags used for the fds passed into QEMU need
  to match the flags QEMU uses to open the file. Currently O_WRONLY
  for src and O_RDONLY for dst.

O_DIRECT is not supported on all systems/filesystems, so run the fdset
test without O_DIRECT if that's the case. The migration code should
still work in that scenario.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 tests/qtest/migration-helpers.c |  7 ++-
 tests/qtest/migration-test.c    | 87 +++++++++++++++++++++++++++++++++
 2 files changed, 92 insertions(+), 2 deletions(-)

diff --git a/tests/qtest/migration-helpers.c b/tests/qtest/migration-helpers.c
index 02b92f0cb6..3013094800 100644
--- a/tests/qtest/migration-helpers.c
+++ b/tests/qtest/migration-helpers.c
@@ -302,7 +302,7 @@ char *resolve_machine_version(const char *alias, const char *var1,
 bool probe_o_direct_support(const char *tmpfs)
 {
     g_autofree char *filename = g_strdup_printf("%s/probe-o-direct", tmpfs);
-    int fd, flags = O_CREAT | O_RDWR | O_DIRECT;
+    int fd, flags = O_CREAT | O_RDWR | O_TRUNC | O_DIRECT;
     void *buf;
     ssize_t ret, len;
     uint64_t offset;
@@ -320,9 +320,12 @@ bool probe_o_direct_support(const char *tmpfs)
     len = 0x100000;
     offset = 0x100000;
 
-    buf = g_malloc0(len);
+    buf = aligned_alloc(len, len);
+    g_assert(buf);
+
     ret = pwrite(fd, buf, len, offset);
     unlink(filename);
+    g_free(buf);
 
     if (ret < 0) {
         return false;
diff --git a/tests/qtest/migration-test.c b/tests/qtest/migration-test.c
index 192b8ec993..bb2dd805fc 100644
--- a/tests/qtest/migration-test.c
+++ b/tests/qtest/migration-test.c
@@ -2251,8 +2251,90 @@ static void test_multifd_file_fixed_ram_dio(void)
 
     test_file_common(&args, true);
 }
+
+static void migrate_multifd_fixed_ram_fdset_dio_end(QTestState *from,
+                                                    QTestState *to,
+                                                    void *opaque)
+{
+    QDict *resp;
+    QList *fdsets;
+
+    /*
+     * Check that we removed the fdsets after migration, otherwise a
+     * second migration would fail due to too many fdsets.
+     */
+
+    resp = qtest_qmp(from, "{'execute': 'query-fdsets', "
+                     "'arguments': {}}");
+    g_assert(qdict_haskey(resp, "return"));
+    fdsets = qdict_get_qlist(resp, "return");
+    g_assert(fdsets && qlist_empty(fdsets));
+}
+#endif /* O_DIRECT */
+
+#ifndef _WIN32
+static void *migrate_multifd_fixed_ram_fdset(QTestState *from, QTestState *to)
+{
+    g_autofree char *file = g_strdup_printf("%s/%s", tmpfs, FILE_TEST_FILENAME);
+    int fds[3];
+    int src_flags = O_CREAT | O_WRONLY;
+    int dst_flags = O_CREAT | O_RDONLY;
+
+    /* main outgoing channel: no O_DIRECT */
+    fds[0] = open(file, src_flags, 0660);
+    assert(fds[0] != -1);
+
+#ifdef O_DIRECT
+    src_flags |= O_DIRECT;
 #endif
 
+    /* secondary outgoing channels */
+    fds[1] = open(file, src_flags, 0660);
+    assert(fds[1] != -1);
+
+    qtest_qmp_fds_assert_success(from, &fds[0], 1, "{'execute': 'add-fd', "
+                                 "'arguments': {'fdset-id': 1}}");
+
+    qtest_qmp_fds_assert_success(from, &fds[1], 1, "{'execute': 'add-fd', "
+                                 "'arguments': {'fdset-id': 1}}");
+
+    /* incoming channel */
+    fds[2] = open(file, dst_flags, 0660);
+    assert(fds[2] != -1);
+
+    qtest_qmp_fds_assert_success(to, &fds[2], 1, "{'execute': 'add-fd', "
+                                 "'arguments': {'fdset-id': 1}}");
+
+#ifdef O_DIRECT
+        migrate_multifd_fixed_ram_dio_start(from, to);
+#else
+        migrate_multifd_fixed_ram_start(from, to);
+#endif
+
+    return NULL;
+}
+
+static void test_multifd_file_fixed_ram_fdset(void)
+{
+    g_autofree char *uri = g_strdup_printf("file:/dev/fdset/1,offset=0x100");
+    MigrateCommon args = {
+        .connect_uri = uri,
+        .listen_uri = "defer",
+        .start_hook = migrate_multifd_fixed_ram_fdset,
+#ifdef O_DIRECT
+        .finish_hook = migrate_multifd_fixed_ram_fdset_dio_end,
+#endif
+    };
+
+    if (!probe_o_direct_support(tmpfs)) {
+        g_test_skip("Filesystem does not support O_DIRECT");
+        return;
+    }
+
+    test_file_common(&args, true);
+}
+#endif /* _WIN32 */
+
 static void test_precopy_tcp_plain(void)
 {
     MigrateCommon args = {
@@ -3511,6 +3593,11 @@ int main(int argc, char **argv)
                    test_multifd_file_fixed_ram_dio);
 #endif
 
+#ifndef _WIN32
+    qtest_add_func("/migration/multifd/file/fixed-ram/fdset",
+                   test_multifd_file_fixed_ram_fdset);
+#endif
+
 #ifdef CONFIG_GNUTLS
     qtest_add_func("/migration/precopy/unix/tls/psk",
                    test_precopy_unix_tls_psk);
-- 
2.35.3