[PATCH v2 2/3] util/userfaultfd: Add uffd_open()

[Peter Xu <peterx@redhat.com>]

Add a helper to create the uffd handle.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/qemu/userfaultfd.h   |  8 ++++++++
 migration/postcopy-ram.c     | 11 +++++------
 tests/qtest/migration-test.c |  3 ++-
 util/userfaultfd.c           | 13 +++++++++++--
 4 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/include/qemu/userfaultfd.h b/include/qemu/userfaultfd.h
index 6b74f92792..2101115f70 100644
--- a/include/qemu/userfaultfd.h
+++ b/include/qemu/userfaultfd.h
@@ -17,6 +17,14 @@
 #include "exec/hwaddr.h"
 #include <linux/userfaultfd.h>
 
+/**
+ * uffd_open(): Open an userfaultfd handle for current context.
+ *
+ * @flags: The flags we want to pass in when creating the handle.
+ *
+ * Returns: the uffd handle if >=0, or <0 if error happens.
+ */
+int uffd_open(int flags);
 int uffd_query_features(uint64_t *features);
 int uffd_create_fd(uint64_t features, bool non_blocking);
 void uffd_close_fd(int uffd_fd);
diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index b9a37ef255..0c55df0e52 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -37,6 +37,7 @@
 #include "qemu-file.h"
 #include "yank_functions.h"
 #include "tls.h"
+#include "qemu/userfaultfd.h"
 
 /* Arbitrary limit on size of each discard command,
  * keeps them around ~200 bytes
@@ -226,11 +227,9 @@ static bool receive_ufd_features(uint64_t *features)
     int ufd;
     bool ret = true;
 
-    /* if we are here __NR_userfaultfd should exists */
-    ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
+    ufd = uffd_open(O_CLOEXEC);
     if (ufd == -1) {
-        error_report("%s: syscall __NR_userfaultfd failed: %s", __func__,
-                     strerror(errno));
+        error_report("%s: uffd_open() failed: %s", __func__, strerror(errno));
         return false;
     }
 
@@ -375,7 +374,7 @@ bool postcopy_ram_supported_by_host(MigrationIncomingState *mis)
         goto out;
     }
 
-    ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
+    ufd = uffd_open(O_CLOEXEC);
     if (ufd == -1) {
         error_report("%s: userfaultfd not available: %s", __func__,
                      strerror(errno));
@@ -1160,7 +1159,7 @@ static int postcopy_temp_pages_setup(MigrationIncomingState *mis)
 int postcopy_ram_incoming_setup(MigrationIncomingState *mis)
 {
     /* Open the fd for the kernel to give us userfaults */
-    mis->userfault_fd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+    mis->userfault_fd = uffd_open(O_CLOEXEC | O_NONBLOCK);
     if (mis->userfault_fd == -1) {
         error_report("%s: Failed to open userfault fd: %s", __func__,
                      strerror(errno));
diff --git a/tests/qtest/migration-test.c b/tests/qtest/migration-test.c
index 1dd32c9506..7a5d1922dd 100644
--- a/tests/qtest/migration-test.c
+++ b/tests/qtest/migration-test.c
@@ -62,13 +62,14 @@ static bool uffd_feature_thread_id;
 #include <sys/eventfd.h>
 #include <sys/ioctl.h>
 #include <linux/userfaultfd.h>
+#include "qemu/userfaultfd.h"
 
 static bool ufd_version_check(void)
 {
     struct uffdio_api api_struct;
     uint64_t ioctl_mask;
 
-    int ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
+    int ufd = uffd_open(O_CLOEXEC);
 
     if (ufd == -1) {
         g_test_message("Skipping test: userfaultfd not available");
diff --git a/util/userfaultfd.c b/util/userfaultfd.c
index f1cd6af2b1..9845a2ec81 100644
--- a/util/userfaultfd.c
+++ b/util/userfaultfd.c
@@ -19,6 +19,15 @@
 #include <sys/syscall.h>
 #include <sys/ioctl.h>
 
+int uffd_open(int flags)
+{
+#if defined(__linux__) && defined(__NR_userfaultfd)
+    return syscall(__NR_userfaultfd, flags);
+#else
+    return -EINVAL;
+#endif
+}
+
 /**
  * uffd_query_features: query UFFD features
  *
@@ -32,7 +41,7 @@ int uffd_query_features(uint64_t *features)
     struct uffdio_api api_struct = { 0 };
     int ret = -1;
 
-    uffd_fd = syscall(__NR_userfaultfd, O_CLOEXEC);
+    uffd_fd = uffd_open(O_CLOEXEC);
     if (uffd_fd < 0) {
         trace_uffd_query_features_nosys(errno);
         return -1;
@@ -69,7 +78,7 @@ int uffd_create_fd(uint64_t features, bool non_blocking)
     uint64_t ioctl_mask = BIT(_UFFDIO_REGISTER) | BIT(_UFFDIO_UNREGISTER);
 
     flags = O_CLOEXEC | (non_blocking ? O_NONBLOCK : 0);
-    uffd_fd = syscall(__NR_userfaultfd, flags);
+    uffd_fd = uffd_open(flags);
     if (uffd_fd < 0) {
         trace_uffd_create_fd_nosys(errno);
         return -1;
-- 
2.37.3