util/userfaultfd: Add uffd_open()

Add a helper to create the uffd handle.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/qemu/userfaultfd.h

@@ -13,10 +13,20 @@
 #ifndef USERFAULTFD_H
 #define USERFAULTFD_H
 
+#ifdef CONFIG_LINUX
+
 #include "qemu/osdep.h"
 #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);
@@ -32,4 +42,6 @@ int uffd_wakeup(int uffd_fd, void *addr, uint64_t length);
 int uffd_read_events(int uffd_fd, struct uffd_msg *msgs, int count);
 bool uffd_poll_events(int uffd_fd, int tmo);
 
+#endif /* CONFIG_LINUX */
+
 #endif /* USERFAULTFD_H */

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 = uffd_open(O_CLOEXEC);
-    ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
     if (ufd == -1) {
-        error_report("%s: syscall __NR_userfaultfd failed: %s", __func__,
+        error_report("%s: uffd_open() failed: %s", __func__, strerror(errno));
-                     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));

tests/qtest/migration-test.c

@@ -61,14 +61,14 @@ static bool uffd_feature_thread_id;
 #if defined(__linux__) && defined(__NR_userfaultfd) && defined(CONFIG_EVENTFD)
 #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");

util/userfaultfd.c

@@ -19,6 +19,15 @@
 #include <sys/syscall.h>
 #include <sys/ioctl.h>
 
+int uffd_open(int flags)
+{
+#if 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;