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, 103 insertions, 0 deletions

diff --git a/include/xen/xen.h b/include/xen/xen.h
new file mode 100644
index 000000000..f280c5dcf
--- /dev/null
+++ b/include/xen/xen.h
@@ -0,0 +1,103 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _XEN_XEN_H
+#define _XEN_XEN_H
+
+#include <linux/types.h>
+
+enum xen_domain_type {
+	XEN_NATIVE,		/* running on bare hardware    */
+	XEN_PV_DOMAIN,		/* running in a PV domain      */
+	XEN_HVM_DOMAIN,		/* running in a Xen hvm domain */
+};
+
+#ifdef CONFIG_XEN
+extern enum xen_domain_type xen_domain_type;
+#else
+#define xen_domain_type		XEN_NATIVE
+#endif
+
+#ifdef CONFIG_XEN_PVH
+extern bool xen_pvh;
+#else
+#define xen_pvh			0
+#endif
+
+#ifdef CONFIG_X86
+#include <asm/cpufeature.h>
+
+#define xen_pv_domain()		(cpu_feature_enabled(X86_FEATURE_XENPV))
+#else
+#define xen_pv_domain()		0
+#endif
+
+#define xen_domain()		(xen_domain_type != XEN_NATIVE)
+#define xen_hvm_domain()	(xen_domain_type == XEN_HVM_DOMAIN)
+#define xen_pvh_domain()	(xen_pvh)
+
+extern uint32_t xen_start_flags;
+
+#ifdef CONFIG_XEN_PV
+extern bool xen_pv_pci_possible;
+#else
+#define xen_pv_pci_possible	0
+#endif
+
+#include <xen/interface/hvm/start_info.h>
+extern struct hvm_start_info pvh_start_info;
+void xen_prepare_pvh(void);
+struct pt_regs;
+void xen_pv_evtchn_do_upcall(struct pt_regs *regs);
+
+#ifdef CONFIG_XEN_DOM0
+#include <xen/interface/xen.h>
+#include <asm/xen/hypervisor.h>
+
+#define xen_initial_domain()	(xen_domain() && \
+				 (xen_start_flags & SIF_INITDOMAIN))
+#else  /* !CONFIG_XEN_DOM0 */
+#define xen_initial_domain()	(0)
+#endif	/* CONFIG_XEN_DOM0 */
+
+struct bio_vec;
+struct page;
+
+bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
+		const struct page *page);
+
+#if defined(CONFIG_MEMORY_HOTPLUG) && defined(CONFIG_XEN_BALLOON)
+extern u64 xen_saved_max_mem_size;
+#endif
+
+#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
+extern unsigned long xen_unpopulated_pages;
+int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages);
+void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages);
+#include <linux/ioport.h>
+int arch_xen_unpopulated_init(struct resource **res);
+#else
+#define xen_unpopulated_pages 0UL
+#include <xen/balloon.h>
+static inline int xen_alloc_unpopulated_pages(unsigned int nr_pages,
+		struct page **pages)
+{
+	return xen_alloc_ballooned_pages(nr_pages, pages);
+}
+static inline void xen_free_unpopulated_pages(unsigned int nr_pages,
+		struct page **pages)
+{
+	xen_free_ballooned_pages(nr_pages, pages);
+}
+#endif
+
+#if defined(CONFIG_XEN_DOM0) && defined(CONFIG_ACPI) && defined(CONFIG_X86)
+bool __init xen_processor_present(uint32_t acpi_id);
+#else
+#include <linux/bug.h>
+static inline bool xen_processor_present(uint32_t acpi_id)
+{
+	BUG();
+	return false;
+}
+#endif
+
+#endif	/* _XEN_XEN_H */