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, 275 insertions, 0 deletions

diff --git a/drivers/clocksource/acpi_pm.c b/drivers/clocksource/acpi_pm.c
new file mode 100644
index 000000000..677929372
--- /dev/null
+++ b/drivers/clocksource/acpi_pm.c
@@ -0,0 +1,275 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * linux/drivers/clocksource/acpi_pm.c
+ *
+ * This file contains the ACPI PM based clocksource.
+ *
+ * This code was largely moved from the i386 timer_pm.c file
+ * which was (C) Dominik Brodowski <linux@brodo.de> 2003
+ * and contained the following comments:
+ *
+ * Driver to use the Power Management Timer (PMTMR) available in some
+ * southbridges as primary timing source for the Linux kernel.
+ *
+ * Based on parts of linux/drivers/acpi/hardware/hwtimer.c, timer_pit.c,
+ * timer_hpet.c, and on Arjan van de Ven's implementation for 2.4.
+ */
+
+#include <linux/acpi_pmtmr.h>
+#include <linux/clocksource.h>
+#include <linux/timex.h>
+#include <linux/errno.h>
+#include <linux/init.h>
+#include <linux/pci.h>
+#include <linux/delay.h>
+#include <asm/io.h>
+#include <asm/time.h>
+
+static void *suspend_resume_cb_data;
+
+static void (*suspend_resume_callback)(void *data, bool suspend);
+
+/*
+ * The I/O port the PMTMR resides at.
+ * The location is detected during setup_arch(),
+ * in arch/i386/kernel/acpi/boot.c
+ */
+u32 pmtmr_ioport __read_mostly;
+
+static inline u32 read_pmtmr(void)
+{
+	/* mask the output to 24 bits */
+	return inl(pmtmr_ioport) & ACPI_PM_MASK;
+}
+
+u32 acpi_pm_read_verified(void)
+{
+	u32 v1 = 0, v2 = 0, v3 = 0;
+
+	/*
+	 * It has been reported that because of various broken
+	 * chipsets (ICH4, PIIX4 and PIIX4E) where the ACPI PM clock
+	 * source is not latched, you must read it multiple
+	 * times to ensure a safe value is read:
+	 */
+	do {
+		v1 = read_pmtmr();
+		v2 = read_pmtmr();
+		v3 = read_pmtmr();
+	} while (unlikely((v1 > v2 && v1 < v3) || (v2 > v3 && v2 < v1)
+			  || (v3 > v1 && v3 < v2)));
+
+	return v2;
+}
+
+void acpi_pmtmr_register_suspend_resume_callback(void (*cb)(void *data, bool suspend), void *data)
+{
+	suspend_resume_callback = cb;
+	suspend_resume_cb_data = data;
+}
+EXPORT_SYMBOL_GPL(acpi_pmtmr_register_suspend_resume_callback);
+
+void acpi_pmtmr_unregister_suspend_resume_callback(void)
+{
+	suspend_resume_callback = NULL;
+	suspend_resume_cb_data = NULL;
+}
+EXPORT_SYMBOL_GPL(acpi_pmtmr_unregister_suspend_resume_callback);
+
+static void acpi_pm_suspend(struct clocksource *cs)
+{
+	if (suspend_resume_callback)
+		suspend_resume_callback(suspend_resume_cb_data, true);
+}
+
+static void acpi_pm_resume(struct clocksource *cs)
+{
+	if (suspend_resume_callback)
+		suspend_resume_callback(suspend_resume_cb_data, false);
+}
+
+static u64 acpi_pm_read(struct clocksource *cs)
+{
+	return (u64)read_pmtmr();
+}
+
+static struct clocksource clocksource_acpi_pm = {
+	.name		= "acpi_pm",
+	.rating		= 200,
+	.read		= acpi_pm_read,
+	.mask		= (u64)ACPI_PM_MASK,
+	.flags		= CLOCK_SOURCE_IS_CONTINUOUS | CLOCK_SOURCE_CALIBRATED,
+	.suspend	= acpi_pm_suspend,
+	.resume		= acpi_pm_resume,
+};
+
+
+#ifdef CONFIG_PCI
+static int acpi_pm_good;
+static int __init acpi_pm_good_setup(char *__str)
+{
+	acpi_pm_good = 1;
+	return 1;
+}
+__setup("acpi_pm_good", acpi_pm_good_setup);
+
+static u64 acpi_pm_read_slow(struct clocksource *cs)
+{
+	return (u64)acpi_pm_read_verified();
+}
+
+static inline void acpi_pm_need_workaround(void)
+{
+	clocksource_acpi_pm.read = acpi_pm_read_slow;
+	clocksource_acpi_pm.rating = 120;
+}
+
+/*
+ * PIIX4 Errata:
+ *
+ * The power management timer may return improper results when read.
+ * Although the timer value settles properly after incrementing,
+ * while incrementing there is a 3 ns window every 69.8 ns where the
+ * timer value is indeterminate (a 4.2% chance that the data will be
+ * incorrect when read). As a result, the ACPI free running count up
+ * timer specification is violated due to erroneous reads.
+ */
+static void acpi_pm_check_blacklist(struct pci_dev *dev)
+{
+	if (acpi_pm_good)
+		return;
+
+	/* the bug has been fixed in PIIX4M */
+	if (dev->revision < 3) {
+		pr_warn("* Found PM-Timer Bug on the chipset. Due to workarounds for a bug,\n"
+			"* this clock source is slow. Consider trying other clock sources\n");
+
+		acpi_pm_need_workaround();
+	}
+}
+DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_82371AB_3,
+			acpi_pm_check_blacklist);
+
+static void acpi_pm_check_graylist(struct pci_dev *dev)
+{
+	if (acpi_pm_good)
+		return;
+
+	pr_warn("* The chipset may have PM-Timer Bug. Due to workarounds for a bug,\n"
+		"* this clock source is slow. If you are sure your timer does not have\n"
+		"* this bug, please use \"acpi_pm_good\" to disable the workaround\n");
+
+	acpi_pm_need_workaround();
+}
+DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_82801DB_0,
+			acpi_pm_check_graylist);
+DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_SERVERWORKS, PCI_DEVICE_ID_SERVERWORKS_LE,
+			acpi_pm_check_graylist);
+#endif
+
+#ifndef CONFIG_X86_64
+#include <asm/mach_timer.h>
+#define PMTMR_EXPECTED_RATE \
+  ((CALIBRATE_LATCH * (PMTMR_TICKS_PER_SEC >> 10)) / (PIT_TICK_RATE>>10))
+/*
+ * Some boards have the PMTMR running way too fast. We check
+ * the PMTMR rate against PIT channel 2 to catch these cases.
+ */
+static int verify_pmtmr_rate(void)
+{
+	u64 value1, value2;
+	unsigned long count, delta;
+
+	mach_prepare_counter();
+	value1 = clocksource_acpi_pm.read(&clocksource_acpi_pm);
+	mach_countup(&count);
+	value2 = clocksource_acpi_pm.read(&clocksource_acpi_pm);
+	delta = (value2 - value1) & ACPI_PM_MASK;
+
+	/* Check that the PMTMR delta is within 5% of what we expect */
+	if (delta < (PMTMR_EXPECTED_RATE * 19) / 20 ||
+	    delta > (PMTMR_EXPECTED_RATE * 21) / 20) {
+		pr_info("PM-Timer running at invalid rate: %lu%% of normal - aborting.\n",
+			100UL * delta / PMTMR_EXPECTED_RATE);
+		return -1;
+	}
+
+	return 0;
+}
+#else
+#define verify_pmtmr_rate() (0)
+#endif
+
+/* Number of monotonicity checks to perform during initialization */
+#define ACPI_PM_MONOTONICITY_CHECKS 10
+/* Number of reads we try to get two different values */
+#define ACPI_PM_READ_CHECKS 10000
+
+static int __init init_acpi_pm_clocksource(void)
+{
+	u64 value1, value2;
+	unsigned int i, j = 0;
+
+	if (!pmtmr_ioport)
+		return -ENODEV;
+
+	/* "verify" this timing source: */
+	for (j = 0; j < ACPI_PM_MONOTONICITY_CHECKS; j++) {
+		udelay(100 * j);
+		value1 = clocksource_acpi_pm.read(&clocksource_acpi_pm);
+		for (i = 0; i < ACPI_PM_READ_CHECKS; i++) {
+			value2 = clocksource_acpi_pm.read(&clocksource_acpi_pm);
+			if (value2 == value1)
+				continue;
+			if (value2 > value1)
+				break;
+			if ((value2 < value1) && ((value2) < 0xFFF))
+				break;
+			pr_info("PM-Timer had inconsistent results: %#llx, %#llx - aborting.\n",
+				value1, value2);
+			pmtmr_ioport = 0;
+			return -EINVAL;
+		}
+		if (i == ACPI_PM_READ_CHECKS) {
+			pr_info("PM-Timer failed consistency check  (%#llx) - aborting.\n",
+				value1);
+			pmtmr_ioport = 0;
+			return -ENODEV;
+		}
+	}
+
+	if (verify_pmtmr_rate() != 0){
+		pmtmr_ioport = 0;
+		return -ENODEV;
+	}
+
+	return clocksource_register_hz(&clocksource_acpi_pm, PMTMR_TICKS_PER_SEC);
+}
+
+/* We use fs_initcall because we want the PCI fixups to have run
+ * but we still need to load before device_initcall
+ */
+fs_initcall(init_acpi_pm_clocksource);
+
+/*
+ * Allow an override of the IOPort. Stupid BIOSes do not tell us about
+ * the PMTimer, but we might know where it is.
+ */
+static int __init parse_pmtmr(char *arg)
+{
+	unsigned int base;
+	int ret;
+
+	ret = kstrtouint(arg, 16, &base);
+	if (ret) {
+		pr_warn("PMTMR: invalid 'pmtmr=' value: '%s'\n", arg);
+		return 1;
+	}
+
+	pr_info("PMTMR IOPort override: 0x%04x -> 0x%04x\n", pmtmr_ioport,
+		base);
+	pmtmr_ioport = base;
+
+	return 1;
+}
+__setup("pmtmr=", parse_pmtmr);