[PATCH v4 2/7] riscv: stacktrace: disable KASAN and KCOV instrumentation for stacktrace.o

[PATCH v4 2/7] riscv: stacktrace: disable KASAN and KCOV instrumentation for stacktrace.o

[Wang Han]

KASAN records stack traces for every alloc/free, which means it walks
the unwinder very frequently. Instrumenting the stack trace collection
code itself adds substantial overhead and makes the traces themselves
noisier.

KCOV instruments every basic-block edge. The unwinder is a hot path,
especially with KASAN enabled, so KCOV instrumentation has the same kind
of cost and noise problem here.

Mark stacktrace.o as not KASAN- or KCOV-instrumented, matching the x86
treatment of its stack unwinding code. RISC-V keeps the relevant unwinder
code in stacktrace.o, so a single translation-unit annotation covers the
equivalent scope. This is a prerequisite preference for the upcoming
reliable unwinder, but the change is valid on its own.

Signed-off-by: Wang Han <wanghan@linux.alibaba.com>
---
 arch/riscv/kernel/Makefile | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index cabb99cadfb6..c565a72a36f3 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -44,6 +44,12 @@ CFLAGS_REMOVE_return_address.o	= $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_sbi_ecall.o = $(CC_FLAGS_FTRACE)
 endif
 
+# When KASAN is enabled, a stack trace is recorded for every alloc/free, which
+# can significantly impact performance. Avoid instrumenting the stack trace
+# collection code to minimize this impact.
+KASAN_SANITIZE_stacktrace.o := n
+KCOV_INSTRUMENT_stacktrace.o := n
+
 always-$(KBUILD_BUILTIN) += vmlinux.lds
 
 obj-y	+= head.o
-- 
2.43.0