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, 142 insertions, 0 deletions

diff --git a/tools/perf/util/tsc.c b/tools/perf/util/tsc.c
new file mode 100644
index 000000000..ebf289bf6
--- /dev/null
+++ b/tools/perf/util/tsc.c
@@ -0,0 +1,142 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <errno.h>
+#include <inttypes.h>
+#include <string.h>
+
+#include <linux/compiler.h>
+#include <linux/perf_event.h>
+#include <linux/stddef.h>
+#include <linux/types.h>
+
+#include <asm/barrier.h>
+
+#include "event.h"
+#include "synthetic-events.h"
+#include "debug.h"
+#include "tsc.h"
+
+u64 perf_time_to_tsc(u64 ns, struct perf_tsc_conversion *tc)
+{
+	u64 t, quot, rem;
+
+	t = ns - tc->time_zero;
+	quot = t / tc->time_mult;
+	rem  = t % tc->time_mult;
+	return (quot << tc->time_shift) +
+	       (rem << tc->time_shift) / tc->time_mult;
+}
+
+u64 tsc_to_perf_time(u64 cyc, struct perf_tsc_conversion *tc)
+{
+	u64 quot, rem;
+
+	if (tc->cap_user_time_short)
+		cyc = tc->time_cycles +
+			((cyc - tc->time_cycles) & tc->time_mask);
+
+	quot = cyc >> tc->time_shift;
+	rem  = cyc & (((u64)1 << tc->time_shift) - 1);
+	return tc->time_zero + quot * tc->time_mult +
+	       ((rem * tc->time_mult) >> tc->time_shift);
+}
+
+int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc,
+			     struct perf_tsc_conversion *tc)
+{
+	u32 seq;
+	int i = 0;
+
+	while (1) {
+		seq = pc->lock;
+		rmb();
+		tc->time_mult = pc->time_mult;
+		tc->time_shift = pc->time_shift;
+		tc->time_zero = pc->time_zero;
+		tc->time_cycles = pc->time_cycles;
+		tc->time_mask = pc->time_mask;
+		tc->cap_user_time_zero = pc->cap_user_time_zero;
+		tc->cap_user_time_short	= pc->cap_user_time_short;
+		rmb();
+		if (pc->lock == seq && !(seq & 1))
+			break;
+		if (++i > 10000) {
+			pr_debug("failed to get perf_event_mmap_page lock\n");
+			return -EINVAL;
+		}
+	}
+
+	if (!tc->cap_user_time_zero)
+		return -EOPNOTSUPP;
+
+	return 0;
+}
+
+int perf_event__synth_time_conv(const struct perf_event_mmap_page *pc,
+				const struct perf_tool *tool,
+				perf_event__handler_t process,
+				struct machine *machine)
+{
+	union perf_event event = {
+		.time_conv = {
+			.header = {
+				.type = PERF_RECORD_TIME_CONV,
+				.size = sizeof(struct perf_record_time_conv),
+			},
+		},
+	};
+	struct perf_tsc_conversion tc;
+	int err;
+
+	if (!pc)
+		return 0;
+	err = perf_read_tsc_conversion(pc, &tc);
+	if (err == -EOPNOTSUPP)
+		return 0;
+	if (err)
+		return err;
+
+	pr_debug2("Synthesizing TSC conversion information\n");
+
+	event.time_conv.time_mult  = tc.time_mult;
+	event.time_conv.time_shift = tc.time_shift;
+	event.time_conv.time_zero  = tc.time_zero;
+	event.time_conv.time_cycles = tc.time_cycles;
+	event.time_conv.time_mask = tc.time_mask;
+	event.time_conv.cap_user_time_zero = tc.cap_user_time_zero;
+	event.time_conv.cap_user_time_short = tc.cap_user_time_short;
+
+	return process(tool, &event, NULL, machine);
+}
+
+u64 __weak rdtsc(void)
+{
+	return 0;
+}
+
+size_t perf_event__fprintf_time_conv(union perf_event *event, FILE *fp)
+{
+	struct perf_record_time_conv *tc = (struct perf_record_time_conv *)event;
+	size_t ret;
+
+	ret  = fprintf(fp, "\n... Time Shift      %" PRI_lu64 "\n", tc->time_shift);
+	ret += fprintf(fp, "... Time Multiplier %" PRI_lu64 "\n", tc->time_mult);
+	ret += fprintf(fp, "... Time Zero       %" PRI_lu64 "\n", tc->time_zero);
+
+	/*
+	 * The event TIME_CONV was extended for the fields from "time_cycles"
+	 * when supported cap_user_time_short, for backward compatibility,
+	 * prints the extended fields only if they are contained in the event.
+	 */
+	if (event_contains(*tc, cap_user_time_short)) {
+		ret += fprintf(fp, "... Time Cycles     %" PRI_lu64 "\n",
+			       tc->time_cycles);
+		ret += fprintf(fp, "... Time Mask       %#" PRI_lx64 "\n",
+			       tc->time_mask);
+		ret += fprintf(fp, "... Cap Time Zero   %" PRId32 "\n",
+			       tc->cap_user_time_zero);
+		ret += fprintf(fp, "... Cap Time Short  %" PRId32 "\n",
+			       tc->cap_user_time_short);
+	}
+
+	return ret;
+}