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, 463 insertions, 0 deletions

diff --git a/arch/xtensa/mm/misc.S b/arch/xtensa/mm/misc.S
new file mode 100644
index 000000000..ec36f73c4
--- /dev/null
+++ b/arch/xtensa/mm/misc.S
@@ -0,0 +1,463 @@
+/*
+ * arch/xtensa/mm/misc.S
+ *
+ * Miscellaneous assembly functions.
+ *
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ *
+ * Copyright (C) 2001 - 2007 Tensilica Inc.
+ *
+ * Chris Zankel	<chris@zankel.net>
+ */
+
+
+#include <linux/linkage.h>
+#include <linux/pgtable.h>
+#include <asm/page.h>
+#include <asm/asmmacro.h>
+#include <asm/cacheasm.h>
+#include <asm/tlbflush.h>
+
+
+/*
+ * clear_page and clear_user_page are the same for non-cache-aliased configs.
+ *
+ * clear_page (unsigned long page)
+ *                    a2
+ */
+
+ENTRY(clear_page)
+
+	abi_entry_default
+
+	movi	a3, 0
+	__loopi	a2, a7, PAGE_SIZE, 32
+	s32i	a3, a2, 0
+	s32i	a3, a2, 4
+	s32i	a3, a2, 8
+	s32i	a3, a2, 12
+	s32i	a3, a2, 16
+	s32i	a3, a2, 20
+	s32i	a3, a2, 24
+	s32i	a3, a2, 28
+	__endla	a2, a7, 32
+
+	abi_ret_default
+
+ENDPROC(clear_page)
+EXPORT_SYMBOL(clear_page)
+
+/*
+ * copy_page and copy_user_page are the same for non-cache-aliased configs.
+ *
+ * copy_page (void *to, void *from)
+ *               a2          a3
+ */
+
+ENTRY(copy_page)
+
+	abi_entry_default
+
+	__loopi a2, a4, PAGE_SIZE, 32
+
+	l32i    a8, a3, 0
+	l32i    a9, a3, 4
+	s32i    a8, a2, 0
+	s32i    a9, a2, 4
+
+	l32i    a8, a3, 8
+	l32i    a9, a3, 12
+	s32i    a8, a2, 8
+	s32i    a9, a2, 12
+
+	l32i    a8, a3, 16
+	l32i    a9, a3, 20
+	s32i    a8, a2, 16
+	s32i    a9, a2, 20
+
+	l32i    a8, a3, 24
+	l32i    a9, a3, 28
+	s32i    a8, a2, 24
+	s32i    a9, a2, 28
+
+	addi    a2, a2, 32
+	addi    a3, a3, 32
+
+	__endl  a2, a4
+
+	abi_ret_default
+
+ENDPROC(copy_page)
+EXPORT_SYMBOL(copy_page)
+
+#ifdef CONFIG_MMU
+/*
+ * If we have to deal with cache aliasing, we use temporary memory mappings
+ * to ensure that the source and destination pages have the same color as
+ * the virtual address. We use way 0 and 1 for temporary mappings in such cases.
+ *
+ * The temporary DTLB entries shouldn't be flushed by interrupts, but are
+ * flushed by preemptive task switches. Special code in the 
+ * fast_second_level_miss handler re-established the temporary mapping. 
+ * It requires that the PPNs for the destination and source addresses are
+ * in a6, and a7, respectively.
+ */
+
+/* TLB miss exceptions are treated special in the following region */
+
+ENTRY(__tlbtemp_mapping_start)
+
+#if (DCACHE_WAY_SIZE > PAGE_SIZE)
+
+/*
+ * clear_page_alias(void *addr, unsigned long paddr)
+ *                     a2              a3
+ */
+
+ENTRY(clear_page_alias)
+
+	abi_entry_default
+
+	movi	a5, PAGE_OFFSET
+	addi	a6, a3, (PAGE_KERNEL | _PAGE_HW_WRITE)
+	mov	a4, a2
+	wdtlb	a6, a2
+	dsync
+
+	movi	a3, 0
+	__loopi	a2, a7, PAGE_SIZE, 32
+	s32i	a3, a2, 0
+	s32i	a3, a2, 4
+	s32i	a3, a2, 8
+	s32i	a3, a2, 12
+	s32i	a3, a2, 16
+	s32i	a3, a2, 20
+	s32i	a3, a2, 24
+	s32i	a3, a2, 28
+	__endla	a2, a7, 32
+
+	/* We need to invalidate the temporary dtlb entry. */
+
+	idtlb	a4
+	dsync
+
+	abi_ret_default
+
+ENDPROC(clear_page_alias)
+
+/*
+ * copy_page_alias(void *to, void *from,
+ *			a2	  a3
+ *                 unsigned long to_paddr, unsigned long from_paddr)
+ *	        		 a4			 a5
+ */
+
+ENTRY(copy_page_alias)
+
+	abi_entry_default
+
+	/* Setup a temporary DTLB for destination. */
+
+	addi	a6, a4, (PAGE_KERNEL | _PAGE_HW_WRITE)
+	wdtlb	a6, a2
+	dsync
+
+	/* Setup a temporary DTLB for source. */
+
+	addi	a7, a5, PAGE_KERNEL
+	addi	a8, a3, 1				# way1
+
+	wdtlb	a7, a8
+	dsync
+
+1:	__loopi a2, a4, PAGE_SIZE, 32
+
+	l32i    a8, a3, 0
+	l32i    a9, a3, 4
+	s32i    a8, a2, 0
+	s32i    a9, a2, 4
+
+	l32i    a8, a3, 8
+	l32i    a9, a3, 12
+	s32i    a8, a2, 8
+	s32i    a9, a2, 12
+
+	l32i    a8, a3, 16
+	l32i    a9, a3, 20
+	s32i    a8, a2, 16
+	s32i    a9, a2, 20
+
+	l32i    a8, a3, 24
+	l32i    a9, a3, 28
+	s32i    a8, a2, 24
+	s32i    a9, a2, 28
+
+	addi    a2, a2, 32
+	addi    a3, a3, 32
+
+	__endl  a2, a4
+
+	/* We need to invalidate any temporary mapping! */
+
+	addi	a2, a2, -PAGE_SIZE
+	idtlb	a2
+	dsync
+
+	addi	a3, a3, -PAGE_SIZE+1
+	idtlb	a3
+	dsync
+
+	abi_ret_default
+
+ENDPROC(copy_page_alias)
+
+#endif
+
+#if (DCACHE_WAY_SIZE > PAGE_SIZE)
+
+/*
+ * void __flush_invalidate_dcache_page_alias (addr, phys)
+ *                                             a2    a3
+ */
+
+ENTRY(__flush_invalidate_dcache_page_alias)
+
+	abi_entry_default
+
+	movi	a7, 0			# required for exception handler
+	addi	a6, a3, (PAGE_KERNEL | _PAGE_HW_WRITE)
+	mov	a4, a2
+	wdtlb	a6, a2
+	dsync
+
+	___flush_invalidate_dcache_page a2 a3
+
+	idtlb	a4
+	dsync
+
+	abi_ret_default
+
+ENDPROC(__flush_invalidate_dcache_page_alias)
+
+/*
+ * void __invalidate_dcache_page_alias (addr, phys)
+ *                                       a2    a3
+ */
+
+ENTRY(__invalidate_dcache_page_alias)
+
+	abi_entry_default
+
+	movi	a7, 0			# required for exception handler
+	addi	a6, a3, (PAGE_KERNEL | _PAGE_HW_WRITE)
+	mov	a4, a2
+	wdtlb	a6, a2
+	dsync
+
+	___invalidate_dcache_page a2 a3
+
+	idtlb	a4
+	dsync
+
+	abi_ret_default
+
+ENDPROC(__invalidate_dcache_page_alias)
+#endif
+
+ENTRY(__tlbtemp_mapping_itlb)
+
+#if (ICACHE_WAY_SIZE > PAGE_SIZE)
+	
+ENTRY(__invalidate_icache_page_alias)
+
+	abi_entry_default
+
+	addi	a6, a3, (PAGE_KERNEL_EXEC | _PAGE_HW_WRITE)
+	mov	a4, a2
+	witlb	a6, a2
+	isync
+
+	___invalidate_icache_page a2 a3
+
+	iitlb	a4
+	isync
+	abi_ret_default
+
+ENDPROC(__invalidate_icache_page_alias)
+
+#endif
+
+/* End of special treatment in tlb miss exception */
+
+ENTRY(__tlbtemp_mapping_end)
+
+#endif /* CONFIG_MMU
+
+/*
+ * void __invalidate_icache_page(ulong start)
+ */
+
+ENTRY(__invalidate_icache_page)
+
+	abi_entry_default
+
+	___invalidate_icache_page a2 a3
+	isync
+
+	abi_ret_default
+
+ENDPROC(__invalidate_icache_page)
+
+/*
+ * void __invalidate_dcache_page(ulong start)
+ */
+
+ENTRY(__invalidate_dcache_page)
+
+	abi_entry_default
+
+	___invalidate_dcache_page a2 a3
+	dsync
+
+	abi_ret_default
+
+ENDPROC(__invalidate_dcache_page)
+
+/*
+ * void __flush_invalidate_dcache_page(ulong start)
+ */
+
+ENTRY(__flush_invalidate_dcache_page)
+
+	abi_entry_default
+
+	___flush_invalidate_dcache_page a2 a3
+
+	dsync
+	abi_ret_default
+
+ENDPROC(__flush_invalidate_dcache_page)
+
+/*
+ * void __flush_dcache_page(ulong start)
+ */
+
+ENTRY(__flush_dcache_page)
+
+	abi_entry_default
+
+	___flush_dcache_page a2 a3
+
+	dsync
+	abi_ret_default
+
+ENDPROC(__flush_dcache_page)
+
+/*
+ * void __invalidate_icache_range(ulong start, ulong size)
+ */
+
+ENTRY(__invalidate_icache_range)
+
+	abi_entry_default
+
+	___invalidate_icache_range a2 a3 a4
+	isync
+
+	abi_ret_default
+
+ENDPROC(__invalidate_icache_range)
+EXPORT_SYMBOL(__invalidate_icache_range)
+
+/*
+ * void __flush_invalidate_dcache_range(ulong start, ulong size)
+ */
+
+ENTRY(__flush_invalidate_dcache_range)
+
+	abi_entry_default
+
+	___flush_invalidate_dcache_range a2 a3 a4
+	dsync
+
+	abi_ret_default
+
+ENDPROC(__flush_invalidate_dcache_range)
+
+/*
+ * void _flush_dcache_range(ulong start, ulong size)
+ */
+
+ENTRY(__flush_dcache_range)
+
+	abi_entry_default
+
+	___flush_dcache_range a2 a3 a4
+	dsync
+
+	abi_ret_default
+
+ENDPROC(__flush_dcache_range)
+EXPORT_SYMBOL(__flush_dcache_range)
+
+/*
+ * void _invalidate_dcache_range(ulong start, ulong size)
+ */
+
+ENTRY(__invalidate_dcache_range)
+
+	abi_entry_default
+
+	___invalidate_dcache_range a2 a3 a4
+
+	abi_ret_default
+
+ENDPROC(__invalidate_dcache_range)
+EXPORT_SYMBOL(__invalidate_dcache_range)
+
+/*
+ * void _invalidate_icache_all(void)
+ */
+
+ENTRY(__invalidate_icache_all)
+
+	abi_entry_default
+
+	___invalidate_icache_all a2 a3
+	isync
+
+	abi_ret_default
+
+ENDPROC(__invalidate_icache_all)
+
+/*
+ * void _flush_invalidate_dcache_all(void)
+ */
+
+ENTRY(__flush_invalidate_dcache_all)
+
+	abi_entry_default
+
+	___flush_invalidate_dcache_all a2 a3
+	dsync
+
+	abi_ret_default
+
+ENDPROC(__flush_invalidate_dcache_all)
+
+/*
+ * void _invalidate_dcache_all(void)
+ */
+
+ENTRY(__invalidate_dcache_all)
+
+	abi_entry_default
+
+	___invalidate_dcache_all a2 a3
+	dsync
+
+	abi_ret_default
+
+ENDPROC(__invalidate_dcache_all)