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, 227 insertions, 0 deletions

diff --git a/include/soc/arc/arc_aux.h b/include/soc/arc/arc_aux.h
new file mode 100644
index 000000000..9c2eff614
--- /dev/null
+++ b/include/soc/arc/arc_aux.h
@@ -0,0 +1,59 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2016-2017 Synopsys, Inc. (www.synopsys.com)
+ */
+
+#ifndef __SOC_ARC_AUX_H__
+#define __SOC_ARC_AUX_H__
+
+#ifdef CONFIG_ARC
+
+#define read_aux_reg(r)		__builtin_arc_lr(r)
+
+/* gcc builtin sr needs reg param to be long immediate */
+#define write_aux_reg(r, v)	__builtin_arc_sr((unsigned int)(v), r)
+
+#else	/* !CONFIG_ARC */
+
+static inline int read_aux_reg(u32 r)
+{
+	return 0;
+}
+
+/*
+ * function helps elide unused variable warning
+ * see: https://lists.infradead.org/pipermail/linux-snps-arc/2016-November/001748.html
+ */
+static inline void write_aux_reg(u32 r, u32 v)
+{
+	;
+}
+
+#endif
+
+#define READ_BCR(reg, into)				\
+{							\
+	unsigned int tmp;				\
+	tmp = read_aux_reg(reg);			\
+	if (sizeof(tmp) == sizeof(into)) {		\
+		into = *((typeof(into) *)&tmp);		\
+	} else {					\
+		extern void bogus_undefined(void);	\
+		bogus_undefined();			\
+	}						\
+}
+
+#define WRITE_AUX(reg, into)				\
+{							\
+	unsigned int tmp;				\
+	if (sizeof(tmp) == sizeof(into)) {		\
+		tmp = (*(unsigned int *)&(into));	\
+		write_aux_reg(reg, tmp);		\
+	} else  {					\
+		extern void bogus_undefined(void);	\
+		bogus_undefined();			\
+	}						\
+}
+
+
+#endif
diff --git a/include/soc/arc/mcip.h b/include/soc/arc/mcip.h
new file mode 100644
index 000000000..a78dacd14
--- /dev/null
+++ b/include/soc/arc/mcip.h
@@ -0,0 +1,133 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * ARConnect IP Support (Multi core enabler: Cross core IPI, RTC ...)
+ *
+ * Copyright (C) 2014-15 Synopsys, Inc. (www.synopsys.com)
+ */
+
+#ifndef __SOC_ARC_MCIP_H
+#define __SOC_ARC_MCIP_H
+
+#include <soc/arc/arc_aux.h>
+
+#define ARC_REG_MCIP_BCR	0x0d0
+#define ARC_REG_MCIP_IDU_BCR	0x0D5
+#define ARC_REG_GFRC_BUILD	0x0D6
+#define ARC_REG_MCIP_CMD	0x600
+#define ARC_REG_MCIP_WDATA	0x601
+#define ARC_REG_MCIP_READBACK	0x602
+
+struct mcip_cmd {
+#ifdef CONFIG_CPU_BIG_ENDIAN
+	unsigned int pad:8, param:16, cmd:8;
+#else
+	unsigned int cmd:8, param:16, pad:8;
+#endif
+
+#define CMD_INTRPT_GENERATE_IRQ		0x01
+#define CMD_INTRPT_GENERATE_ACK		0x02
+#define CMD_INTRPT_READ_STATUS		0x03
+#define CMD_INTRPT_CHECK_SOURCE		0x04
+
+/* Semaphore Commands */
+#define CMD_SEMA_CLAIM_AND_READ		0x11
+#define CMD_SEMA_RELEASE		0x12
+
+#define CMD_DEBUG_SET_MASK		0x34
+#define CMD_DEBUG_READ_MASK		0x35
+#define CMD_DEBUG_SET_SELECT		0x36
+#define CMD_DEBUG_READ_SELECT		0x37
+
+#define CMD_GFRC_READ_LO		0x42
+#define CMD_GFRC_READ_HI		0x43
+#define CMD_GFRC_SET_CORE		0x47
+#define CMD_GFRC_READ_CORE		0x48
+
+#define CMD_IDU_ENABLE			0x71
+#define CMD_IDU_DISABLE			0x72
+#define CMD_IDU_SET_MODE		0x74
+#define CMD_IDU_READ_MODE		0x75
+#define CMD_IDU_SET_DEST		0x76
+#define CMD_IDU_ACK_CIRQ		0x79
+#define CMD_IDU_SET_MASK		0x7C
+
+#define IDU_M_TRIG_LEVEL		0x0
+#define IDU_M_TRIG_EDGE			0x1
+
+#define IDU_M_DISTRI_RR			0x0
+#define IDU_M_DISTRI_DEST		0x2
+};
+
+struct mcip_bcr {
+#ifdef CONFIG_CPU_BIG_ENDIAN
+		unsigned int pad4:6, pw_dom:1, pad3:1,
+			     idu:1, pad2:1, num_cores:6,
+			     pad:1,  gfrc:1, dbg:1, pw:1,
+			     msg:1, sem:1, ipi:1, slv:1,
+			     ver:8;
+#else
+		unsigned int ver:8,
+			     slv:1, ipi:1, sem:1, msg:1,
+			     pw:1, dbg:1, gfrc:1, pad:1,
+			     num_cores:6, pad2:1, idu:1,
+			     pad3:1, pw_dom:1, pad4:6;
+#endif
+};
+
+struct mcip_idu_bcr {
+#ifdef CONFIG_CPU_BIG_ENDIAN
+	unsigned int pad:21, cirqnum:3, ver:8;
+#else
+	unsigned int ver:8, cirqnum:3, pad:21;
+#endif
+};
+
+
+/*
+ * Build register for IDU contains not an actual number of supported common
+ * interrupts but an exponent of 2 which must be multiplied by 4 to
+ * get a number of supported common interrupts.
+ */
+#define mcip_idu_bcr_to_nr_irqs(bcr) (4 * (1 << (bcr).cirqnum))
+
+/*
+ * MCIP programming model
+ *
+ * - Simple commands write {cmd:8,param:16} to MCIP_CMD aux reg
+ *   (param could be irq, common_irq, core_id ...)
+ * - More involved commands setup MCIP_WDATA with cmd specific data
+ *   before invoking the simple command
+ */
+static inline void __mcip_cmd(unsigned int cmd, unsigned int param)
+{
+	struct mcip_cmd buf;
+
+	buf.pad = 0;
+	buf.cmd = cmd;
+	buf.param = param;
+
+	WRITE_AUX(ARC_REG_MCIP_CMD, buf);
+}
+
+/*
+ * Setup additional data for a cmd
+ * Callers need to lock to ensure atomicity
+ */
+static inline void __mcip_cmd_data(unsigned int cmd, unsigned int param,
+				   unsigned int data)
+{
+	write_aux_reg(ARC_REG_MCIP_WDATA, data);
+
+	__mcip_cmd(cmd, param);
+}
+
+/*
+ * Read MCIP register
+ */
+static inline unsigned int __mcip_cmd_read(unsigned int cmd, unsigned int param)
+{
+	__mcip_cmd(cmd, param);
+	return read_aux_reg(ARC_REG_MCIP_READBACK);
+}
+
+#endif
diff --git a/include/soc/arc/timers.h b/include/soc/arc/timers.h
new file mode 100644
index 000000000..51a741662
--- /dev/null
+++ b/include/soc/arc/timers.h
@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2016-17 Synopsys, Inc. (www.synopsys.com)
+ */
+
+#ifndef __SOC_ARC_TIMERS_H
+#define __SOC_ARC_TIMERS_H
+
+#include <soc/arc/arc_aux.h>
+
+/* Timer related Aux registers */
+#define ARC_REG_TIMER0_LIMIT	0x23	/* timer 0 limit */
+#define ARC_REG_TIMER0_CTRL	0x22	/* timer 0 control */
+#define ARC_REG_TIMER0_CNT	0x21	/* timer 0 count */
+#define ARC_REG_TIMER1_LIMIT	0x102	/* timer 1 limit */
+#define ARC_REG_TIMER1_CTRL	0x101	/* timer 1 control */
+#define ARC_REG_TIMER1_CNT	0x100	/* timer 1 count */
+
+/* CTRL reg bits */
+#define ARC_TIMER_CTRL_IE	(1 << 0) /* Interrupt when Count reaches limit */
+#define ARC_TIMER_CTRL_NH	(1 << 1) /* Count only when CPU NOT halted */
+
+#define ARC_TIMERN_MAX		0xFFFFFFFF
+
+#define ARC_REG_TIMERS_BCR	0x75
+
+struct bcr_timer {
+#ifdef CONFIG_CPU_BIG_ENDIAN
+	unsigned int pad2:15, rtsc:1, pad1:5, rtc:1, t1:1, t0:1, ver:8;
+#else
+	unsigned int ver:8, t0:1, t1:1, rtc:1, pad1:5, rtsc:1, pad2:15;
+#endif
+};
+
+#endif