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, 31 insertions, 0 deletions

diff --git a/include/clocksource/timer-goldfish.h b/include/clocksource/timer-goldfish.h
new file mode 100644
index 000000000..05a3a4f61
--- /dev/null
+++ b/include/clocksource/timer-goldfish.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * goldfish-timer clocksource
+ * Registers definition for the goldfish-timer device
+ */
+
+#ifndef _CLOCKSOURCE_TIMER_GOLDFISH_H
+#define _CLOCKSOURCE_TIMER_GOLDFISH_H
+
+/*
+ * TIMER_TIME_LOW	 get low bits of current time and update TIMER_TIME_HIGH
+ * TIMER_TIME_HIGH	 get high bits of time at last TIMER_TIME_LOW read
+ * TIMER_ALARM_LOW	 set low bits of alarm and activate it
+ * TIMER_ALARM_HIGH	 set high bits of next alarm
+ * TIMER_IRQ_ENABLED	 enable alarm interrupt
+ * TIMER_CLEAR_ALARM	 disarm an existing alarm
+ * TIMER_ALARM_STATUS	 alarm status (running or not)
+ * TIMER_CLEAR_INTERRUPT clear interrupt
+ */
+#define TIMER_TIME_LOW		0x00
+#define TIMER_TIME_HIGH		0x04
+#define TIMER_ALARM_LOW		0x08
+#define TIMER_ALARM_HIGH	0x0c
+#define TIMER_IRQ_ENABLED	0x10
+#define TIMER_CLEAR_ALARM	0x14
+#define TIMER_ALARM_STATUS	0x18
+#define TIMER_CLEAR_INTERRUPT	0x1c
+
+extern int goldfish_timer_init(int irq, void __iomem *base);
+
+#endif /* _CLOCKSOURCE_TIMER_GOLDFISH_H */