KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 110 insertions, 0 deletions

diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
new file mode 100644
index 000000000..6b2f7208b
--- /dev/null
+++ b/drivers/tty/ttynull.c
@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019 Axis Communications AB
+ *
+ * Based on ttyprintk.c:
+ *  Copyright (C) 2010 Samo Pogacnik
+ */
+
+#include <linux/console.h>
+#include <linux/module.h>
+#include <linux/tty.h>
+
+static const struct tty_port_operations ttynull_port_ops;
+static struct tty_driver *ttynull_driver;
+static struct tty_port ttynull_port;
+
+static int ttynull_open(struct tty_struct *tty, struct file *filp)
+{
+	return tty_port_open(&ttynull_port, tty, filp);
+}
+
+static void ttynull_close(struct tty_struct *tty, struct file *filp)
+{
+	tty_port_close(&ttynull_port, tty, filp);
+}
+
+static void ttynull_hangup(struct tty_struct *tty)
+{
+	tty_port_hangup(&ttynull_port);
+}
+
+static ssize_t ttynull_write(struct tty_struct *tty, const u8 *buf,
+			     size_t count)
+{
+	return count;
+}
+
+static unsigned int ttynull_write_room(struct tty_struct *tty)
+{
+	return 65536;
+}
+
+static const struct tty_operations ttynull_ops = {
+	.open = ttynull_open,
+	.close = ttynull_close,
+	.hangup = ttynull_hangup,
+	.write = ttynull_write,
+	.write_room = ttynull_write_room,
+};
+
+static struct tty_driver *ttynull_device(struct console *c, int *index)
+{
+	*index = 0;
+	return ttynull_driver;
+}
+
+static struct console ttynull_console = {
+	.name = "ttynull",
+	.device = ttynull_device,
+};
+
+static int __init ttynull_init(void)
+{
+	struct tty_driver *driver;
+	int ret;
+
+	driver = tty_alloc_driver(1,
+		TTY_DRIVER_RESET_TERMIOS |
+		TTY_DRIVER_REAL_RAW |
+		TTY_DRIVER_UNNUMBERED_NODE);
+	if (IS_ERR(driver))
+		return PTR_ERR(driver);
+
+	tty_port_init(&ttynull_port);
+	ttynull_port.ops = &ttynull_port_ops;
+
+	driver->driver_name = "ttynull";
+	driver->name = "ttynull";
+	driver->type = TTY_DRIVER_TYPE_CONSOLE;
+	driver->init_termios = tty_std_termios;
+	driver->init_termios.c_oflag = OPOST | OCRNL | ONOCR | ONLRET;
+	tty_set_operations(driver, &ttynull_ops);
+	tty_port_link_device(&ttynull_port, driver, 0);
+
+	ret = tty_register_driver(driver);
+	if (ret < 0) {
+		tty_driver_kref_put(driver);
+		tty_port_destroy(&ttynull_port);
+		return ret;
+	}
+
+	ttynull_driver = driver;
+	register_console(&ttynull_console);
+
+	return 0;
+}
+
+static void __exit ttynull_exit(void)
+{
+	unregister_console(&ttynull_console);
+	tty_unregister_driver(ttynull_driver);
+	tty_driver_kref_put(ttynull_driver);
+	tty_port_destroy(&ttynull_port);
+}
+
+module_init(ttynull_init);
+module_exit(ttynull_exit);
+
+MODULE_DESCRIPTION("NULL TTY driver");
+MODULE_LICENSE("GPL v2");