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>

3 files changed, 367 insertions, 0 deletions

diff --git a/drivers/dax/hmem/Makefile b/drivers/dax/hmem/Makefile
new file mode 100644
index 000000000..d4c4cd6bc
--- /dev/null
+++ b/drivers/dax/hmem/Makefile
@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0
+# device_hmem.o deliberately precedes dax_hmem.o for initcall ordering
+obj-$(CONFIG_DEV_DAX_HMEM_DEVICES) += device_hmem.o
+obj-$(CONFIG_DEV_DAX_HMEM) += dax_hmem.o
+
+device_hmem-y := device.o
+dax_hmem-y := hmem.o
diff --git a/drivers/dax/hmem/device.c b/drivers/dax/hmem/device.c
new file mode 100644
index 000000000..d70359b43
--- /dev/null
+++ b/drivers/dax/hmem/device.c
@@ -0,0 +1,103 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/platform_device.h>
+#include <linux/memregion.h>
+#include <linux/module.h>
+#include <linux/dax.h>
+#include <linux/mm.h>
+#include "../bus.h"
+
+static bool nohmem;
+module_param_named(disable, nohmem, bool, 0444);
+
+static bool platform_initialized;
+static DEFINE_MUTEX(hmem_resource_lock);
+static struct resource hmem_active = {
+	.name = "HMEM devices",
+	.start = 0,
+	.end = -1,
+	.flags = IORESOURCE_MEM,
+};
+
+int walk_hmem_resources(struct device *host, walk_hmem_fn fn)
+{
+	struct resource *res;
+	int rc = 0;
+
+	mutex_lock(&hmem_resource_lock);
+	for (res = hmem_active.child; res; res = res->sibling) {
+		rc = fn(host, (int) res->desc, res);
+		if (rc)
+			break;
+	}
+	mutex_unlock(&hmem_resource_lock);
+	return rc;
+}
+EXPORT_SYMBOL_GPL(walk_hmem_resources);
+
+static void hmem_work(struct work_struct *work)
+{
+	/* place holder until dax_hmem driver attaches */
+}
+
+static struct hmem_platform_device hmem_platform = {
+	.pdev = {
+		.name = "hmem_platform",
+		.id = 0,
+	},
+	.work = __WORK_INITIALIZER(hmem_platform.work, hmem_work),
+};
+
+static void __hmem_register_resource(int target_nid, struct resource *res)
+{
+	struct resource *new;
+	int rc;
+
+	new = __request_region(&hmem_active, res->start, resource_size(res), "",
+			       0);
+	if (!new) {
+		pr_debug("hmem range %pr already active\n", res);
+		return;
+	}
+
+	new->desc = target_nid;
+
+	if (platform_initialized)
+		return;
+
+	rc = platform_device_register(&hmem_platform.pdev);
+	if (rc) {
+		pr_err_once("failed to register device-dax hmem_platform device\n");
+		return;
+	}
+
+	platform_initialized = true;
+}
+
+void hmem_register_resource(int target_nid, struct resource *res)
+{
+	if (nohmem)
+		return;
+
+	mutex_lock(&hmem_resource_lock);
+	__hmem_register_resource(target_nid, res);
+	mutex_unlock(&hmem_resource_lock);
+}
+
+static __init int hmem_register_one(struct resource *res, void *data)
+{
+	hmem_register_resource(phys_to_target_node(res->start), res);
+
+	return 0;
+}
+
+static __init int hmem_init(void)
+{
+	walk_soft_reserve_res(0, -1, NULL, hmem_register_one);
+	return 0;
+}
+
+/*
+ * As this is a fallback for address ranges unclaimed by the ACPI HMAT
+ * parsing it must be at an initcall level greater than hmat_init().
+ */
+device_initcall(hmem_init);
diff --git a/drivers/dax/hmem/hmem.c b/drivers/dax/hmem/hmem.c
new file mode 100644
index 000000000..af21f66bf
--- /dev/null
+++ b/drivers/dax/hmem/hmem.c
@@ -0,0 +1,257 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/platform_device.h>
+#include <linux/memregion.h>
+#include <linux/module.h>
+#include <linux/dax.h>
+#include "../../cxl/cxl.h"
+#include "../bus.h"
+
+static bool region_idle;
+module_param_named(region_idle, region_idle, bool, 0644);
+
+static int dax_hmem_probe(struct platform_device *pdev)
+{
+	unsigned long flags = IORESOURCE_DAX_KMEM;
+	struct device *dev = &pdev->dev;
+	struct dax_region *dax_region;
+	struct memregion_info *mri;
+	struct dev_dax_data data;
+
+	/*
+	 * @region_idle == true indicates that an administrative agent
+	 * wants to manipulate the range partitioning before the devices
+	 * are created, so do not send them to the dax_kmem driver by
+	 * default.
+	 */
+	if (region_idle)
+		flags = 0;
+
+	mri = dev->platform_data;
+	dax_region = alloc_dax_region(dev, pdev->id, &mri->range,
+				      mri->target_node, PMD_SIZE, flags);
+	if (!dax_region)
+		return -ENOMEM;
+
+	data = (struct dev_dax_data) {
+		.dax_region = dax_region,
+		.id = -1,
+		.size = region_idle ? 0 : range_len(&mri->range),
+		.memmap_on_memory = false,
+	};
+
+	return PTR_ERR_OR_ZERO(devm_create_dev_dax(&data));
+}
+
+static struct platform_driver dax_hmem_driver = {
+	.probe = dax_hmem_probe,
+	.driver = {
+		.name = "hmem",
+	},
+};
+
+static void release_memregion(void *data)
+{
+	memregion_free((long) data);
+}
+
+static void release_hmem(void *pdev)
+{
+	platform_device_unregister(pdev);
+}
+
+static struct workqueue_struct *dax_hmem_wq;
+
+void dax_hmem_flush_work(void)
+{
+	flush_workqueue(dax_hmem_wq);
+}
+EXPORT_SYMBOL_FOR_MODULES(dax_hmem_flush_work, "dax_cxl");
+
+static int __hmem_register_device(struct device *host, int target_nid,
+				  const struct resource *res)
+{
+	struct platform_device *pdev;
+	struct memregion_info info;
+	long id;
+	int rc;
+
+	rc = region_intersects_soft_reserve(res->start, resource_size(res));
+	if (rc != REGION_INTERSECTS)
+		return 0;
+
+	/* TODO: Add Soft-Reserved memory back to iomem */
+
+	id = memregion_alloc(GFP_KERNEL);
+	if (id < 0) {
+		dev_err(host, "memregion allocation failure for %pr\n", res);
+		return -ENOMEM;
+	}
+	rc = devm_add_action_or_reset(host, release_memregion, (void *) id);
+	if (rc)
+		return rc;
+
+	pdev = platform_device_alloc("hmem", id);
+	if (!pdev) {
+		dev_err(host, "device allocation failure for %pr\n", res);
+		return -ENOMEM;
+	}
+
+	pdev->dev.parent = host;
+	pdev->dev.numa_node = numa_map_to_online_node(target_nid);
+	info = (struct memregion_info) {
+		.target_node = target_nid,
+		.range = {
+			.start = res->start,
+			.end = res->end,
+		},
+	};
+	rc = platform_device_add_data(pdev, &info, sizeof(info));
+	if (rc < 0) {
+		dev_err(host, "memregion_info allocation failure for %pr\n",
+		       res);
+		goto out_put;
+	}
+
+	rc = platform_device_add(pdev);
+	if (rc < 0) {
+		dev_err(host, "%s add failed for %pr\n", dev_name(&pdev->dev),
+			res);
+		goto out_put;
+	}
+
+	return devm_add_action_or_reset(host, release_hmem, pdev);
+
+out_put:
+	platform_device_put(pdev);
+	return rc;
+}
+
+static int hmem_register_cxl_device(struct device *host, int target_nid,
+				    const struct resource *res)
+{
+	if (region_intersects(res->start, resource_size(res), IORESOURCE_MEM,
+			      IORES_DESC_CXL) == REGION_DISJOINT)
+		return 0;
+
+	if (cxl_region_contains_resource(res)) {
+		dev_dbg(host, "CXL claims resource, dropping: %pr\n", res);
+		return 0;
+	}
+
+	dev_dbg(host, "CXL did not claim resource, registering: %pr\n", res);
+	return __hmem_register_device(host, target_nid, res);
+}
+
+static void process_defer_work(struct work_struct *w)
+{
+	struct hmem_platform_device *hpdev = container_of(w, typeof(*hpdev), work);
+	struct device *dev = &hpdev->pdev.dev;
+
+	/* Relies on cxl_acpi and cxl_pci having had a chance to load */
+	wait_for_device_probe();
+
+	guard(device)(dev);
+	if (!dev->driver)
+		goto out;
+
+	if (!hpdev->did_probe) {
+		hpdev->did_probe = true;
+		walk_hmem_resources(dev, hmem_register_cxl_device);
+	}
+out:
+	put_device(dev);
+}
+
+static int hmem_register_device(struct device *host, int target_nid,
+				const struct resource *res)
+{
+	struct platform_device *pdev = to_platform_device(host);
+	struct hmem_platform_device *hpdev = to_hmem_platform_device(pdev);
+
+	if (IS_ENABLED(CONFIG_DEV_DAX_CXL) &&
+	    region_intersects(res->start, resource_size(res), IORESOURCE_MEM,
+			      IORES_DESC_CXL) != REGION_DISJOINT) {
+		if (!hpdev->did_probe) {
+			dev_dbg(host, "await CXL initial probe: %pr\n", res);
+			hpdev->work.func = process_defer_work;
+			get_device(host);
+			if (!queue_work(dax_hmem_wq, &hpdev->work))
+				put_device(host);
+			return 0;
+		}
+		dev_dbg(host, "deferring range to CXL: %pr\n", res);
+		return 0;
+	}
+
+	return __hmem_register_device(host, target_nid, res);
+}
+
+static int dax_hmem_platform_probe(struct platform_device *pdev)
+{
+	struct hmem_platform_device *hpdev = to_hmem_platform_device(pdev);
+
+	/* queue is only flushed on module unload, fail rebind with pending work */
+	if (work_pending(&hpdev->work))
+		return -EBUSY;
+
+	return walk_hmem_resources(&pdev->dev, hmem_register_device);
+}
+
+static struct platform_driver dax_hmem_platform_driver = {
+	.probe = dax_hmem_platform_probe,
+	.driver = {
+		.name = "hmem_platform",
+	},
+};
+
+static __init int dax_hmem_init(void)
+{
+	int rc;
+
+	/*
+	 * Ensure that cxl_acpi and cxl_pci have a chance to kick off
+	 * CXL topology discovery at least once before scanning the
+	 * iomem resource tree for IORES_DESC_CXL resources.
+	 */
+	if (IS_ENABLED(CONFIG_DEV_DAX_CXL)) {
+		request_module("cxl_acpi");
+		request_module("cxl_pci");
+	}
+
+	dax_hmem_wq = alloc_ordered_workqueue("dax_hmem_wq", 0);
+	if (!dax_hmem_wq)
+		return -ENOMEM;
+
+	rc = platform_driver_register(&dax_hmem_platform_driver);
+	if (rc)
+		goto err_platform_driver;
+
+	rc = platform_driver_register(&dax_hmem_driver);
+	if (rc)
+		goto err_driver;
+
+	return 0;
+
+err_driver:
+	platform_driver_unregister(&dax_hmem_platform_driver);
+err_platform_driver:
+	destroy_workqueue(dax_hmem_wq);
+
+	return rc;
+}
+
+static __exit void dax_hmem_exit(void)
+{
+	platform_driver_unregister(&dax_hmem_driver);
+	platform_driver_unregister(&dax_hmem_platform_driver);
+	destroy_workqueue(dax_hmem_wq);
+}
+
+module_init(dax_hmem_init);
+module_exit(dax_hmem_exit);
+
+MODULE_ALIAS("platform:hmem*");
+MODULE_ALIAS("platform:hmem_platform*");
+MODULE_DESCRIPTION("HMEM DAX: direct access to 'specific purpose' memory");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Intel Corporation");