kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 121 insertions, 0 deletions

diff --git a/drivers/power/reset/qemu-virt-ctrl.c b/drivers/power/reset/qemu-virt-ctrl.c
new file mode 100644
index 000000000..97875007f
--- /dev/null
+++ b/drivers/power/reset/qemu-virt-ctrl.c
@@ -0,0 +1,121 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * QEMU Virt Machine System Controller Driver
+ *
+ * Copyright (C) 2026 Kuan-Wei Chiu <visitorckw@gmail.com>
+ */
+
+#include <linux/io.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/reboot.h>
+
+/* Registers */
+#define VIRT_CTRL_REG_FEATURES	0x00
+#define VIRT_CTRL_REG_CMD	0x04
+
+/* Commands */
+#define CMD_NOOP	0
+#define CMD_RESET	1
+#define CMD_HALT	2
+#define CMD_PANIC	3
+
+struct qemu_virt_ctrl {
+	void __iomem *base;
+	struct notifier_block reboot_nb;
+};
+
+static inline void virt_ctrl_write32(u32 val, void __iomem *addr)
+{
+	if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN))
+		iowrite32be(val, addr);
+	else
+		iowrite32(val, addr);
+}
+
+static int qemu_virt_ctrl_power_off(struct sys_off_data *data)
+{
+	struct qemu_virt_ctrl *ctrl = data->cb_data;
+
+	virt_ctrl_write32(CMD_HALT, ctrl->base + VIRT_CTRL_REG_CMD);
+
+	return NOTIFY_DONE;
+}
+
+static int qemu_virt_ctrl_restart(struct sys_off_data *data)
+{
+	struct qemu_virt_ctrl *ctrl = data->cb_data;
+
+	virt_ctrl_write32(CMD_RESET, ctrl->base + VIRT_CTRL_REG_CMD);
+
+	return NOTIFY_DONE;
+}
+
+static int qemu_virt_ctrl_reboot_notify(struct notifier_block *nb,
+					unsigned long action, void *data)
+{
+	struct qemu_virt_ctrl *ctrl = container_of(nb, struct qemu_virt_ctrl, reboot_nb);
+
+	if (action == SYS_HALT)
+		virt_ctrl_write32(CMD_HALT, ctrl->base + VIRT_CTRL_REG_CMD);
+
+	return NOTIFY_DONE;
+}
+
+static int qemu_virt_ctrl_probe(struct platform_device *pdev)
+{
+	struct qemu_virt_ctrl *ctrl;
+	int ret;
+
+	ctrl = devm_kzalloc(&pdev->dev, sizeof(*ctrl), GFP_KERNEL);
+	if (!ctrl)
+		return -ENOMEM;
+
+	ctrl->base = devm_platform_ioremap_resource(pdev, 0);
+	if (IS_ERR(ctrl->base))
+		return PTR_ERR(ctrl->base);
+
+	ret = devm_register_sys_off_handler(&pdev->dev,
+					    SYS_OFF_MODE_RESTART,
+					    SYS_OFF_PRIO_DEFAULT,
+					    qemu_virt_ctrl_restart,
+					    ctrl);
+	if (ret)
+		return dev_err_probe(&pdev->dev, ret,
+				     "cannot register restart handler\n");
+
+	ret = devm_register_sys_off_handler(&pdev->dev,
+					    SYS_OFF_MODE_POWER_OFF,
+					    SYS_OFF_PRIO_DEFAULT,
+					    qemu_virt_ctrl_power_off,
+					    ctrl);
+	if (ret)
+		return dev_err_probe(&pdev->dev, ret,
+				     "cannot register power-off handler\n");
+
+	ctrl->reboot_nb.notifier_call = qemu_virt_ctrl_reboot_notify;
+	ret = devm_register_reboot_notifier(&pdev->dev, &ctrl->reboot_nb);
+	if (ret)
+		return dev_err_probe(&pdev->dev, ret, "cannot register reboot notifier\n");
+
+	return 0;
+}
+
+static const struct platform_device_id qemu_virt_ctrl_id[] = {
+	{ .name = "qemu-virt-ctrl" },
+	{ }
+};
+MODULE_DEVICE_TABLE(platform, qemu_virt_ctrl_id);
+
+static struct platform_driver qemu_virt_ctrl_driver = {
+	.probe = qemu_virt_ctrl_probe,
+	.driver = {
+		.name = "qemu-virt-ctrl",
+	},
+	.id_table = qemu_virt_ctrl_id,
+};
+module_platform_driver(qemu_virt_ctrl_driver);
+
+MODULE_AUTHOR("Kuan-Wei Chiu <visitorckw@gmail.com>");
+MODULE_DESCRIPTION("QEMU Virt Machine System Controller Driver");
+MODULE_LICENSE("GPL");