memfd: fix vhost-user-test on non-memfd capable host

On RHEL7, memfd is not supported, and vhost-user-test fails:
TEST: tests/vhost-user-test... (pid=10248)
  /x86_64/vhost-user/migrate:
  qemu-system-x86_64: -object memory-backend-memfd,id=mem,size=2M,: failed to create memfd
FAIL

There is a qemu_memfd_check() to prevent running memfd path, but it
also checks for fallback implementation. Let's specialize
qemu_memfd_check() to check memfd only, while qemu_memfd_alloc_check()
checks for the qemu_memfd_alloc() API.
Reported-by: NMiroslav Rezanina <mrezanin@redhat.com>
Tested-by: NMiroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180328121804.16203-1-marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>

hw/virtio/vhost.c

@@ -1223,7 +1223,7 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
         if (!(hdev->features & (0x1ULL << VHOST_F_LOG_ALL))) {
             error_setg(&hdev->migration_blocker,
                        "Migration disabled: vhost lacks VHOST_F_LOG_ALL feature.");
-        } else if (vhost_dev_log_is_shared(hdev) && !qemu_memfd_check()) {
+        } else if (vhost_dev_log_is_shared(hdev) && !qemu_memfd_alloc_check()) {
             error_setg(&hdev->migration_blocker,
                        "Migration disabled: failed to allocate shared memory");
         }

include/qemu/memfd.h

@@ -18,6 +18,7 @@
 
 int qemu_memfd_create(const char *name, size_t size, bool hugetlb,
                       uint64_t hugetlbsize, unsigned int seals, Error **errp);
+bool qemu_memfd_alloc_check(void);
 void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals,
                        int *fd, Error **errp);
 void qemu_memfd_free(void *ptr, size_t size, int fd);

util/memfd.c

@@ -173,7 +173,13 @@ enum {
     MEMFD_TODO
 };
 
-bool qemu_memfd_check(void)
+/**
+ * qemu_memfd_alloc_check():
+ *
+ * Check if qemu_memfd_alloc() can allocate, including using a
+ * fallback implementation when host doesn't support memfd.
+ */
+bool qemu_memfd_alloc_check(void)
 {
     static int memfd_check = MEMFD_TODO;
 
@@ -188,3 +194,29 @@ bool qemu_memfd_check(void)
 
     return memfd_check == MEMFD_OK;
 }
+
+/**
+ * qemu_memfd_check():
+ *
+ * Check if host supports memfd.
+ */
+bool qemu_memfd_check(void)
+{
+#ifdef CONFIG_LINUX
+    static int memfd_check = MEMFD_TODO;
+
+    if (memfd_check == MEMFD_TODO) {
+        int mfd = memfd_create("test", 0);
+        if (mfd >= 0) {
+            memfd_check = MEMFD_OK;
+            close(mfd);
+        } else {
+            memfd_check = MEMFD_KO;
+        }
+    }
+
+    return memfd_check == MEMFD_OK;
+#else
+    return false;
+#endif
+}