[PATCH v2 19/19] arm64: idle: don't instrument idle code with KCOV

Mark Rutland
2021-05-19T13:11:40Z

The low-level idle code in arch_cpu_idle() and its callees runs at a
time where where portions of the kernel environment aren't available.
For example, RCU may not be watching, and lockdep state may be
out-of-sync with the hardware. Due to this, it is not sound to
instrument this code.

We generally avoid instrumentation by marking the entry functions as
`noinstr`, but currently this doesn't inhibit KCOV instrumentation.
Prevent this by factoring these functions into a new idle.c so that we
can disable KCOV for the entire compilation unit, as is done for the
core idle code in kernel/sched/idle.c.

We'd like to keep instrumentation of the rest of process.c, and for the
existing code in cpuidle.c, so a new compilation unit is preferable. The
arch_cpu_idle_dead() function in process.c is a cpu hotplug function
that is safe to instrument, so it is left as-is in process.c.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
---
arch/arm64/kernel/Makefile | 3 +-
arch/arm64/kernel/idle.c | 69 +++++++++++++++++++++++++++++++++++++++++++++
arch/arm64/kernel/process.c | 57 -------------------------------------
3 files changed, 71 insertions(+), 58 deletions(-)
create mode 100644 arch/arm64/kernel/idle.c

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 294063032428..708df7de84d1 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -15,6 +15,7 @@ CFLAGS_REMOVE_syscall.o = -fstack-protector -fstack-protector-strong
CFLAGS_syscall.o += -fno-stack-protector

KCOV_INSTRUMENT_entry.o := n
+KCOV_INSTRUMENT_idle.o := n

# Object file lists.
obj-y := debug-monitors.o entry.o irq.o fpsimd.o \
@@ -24,7 +25,7 @@ obj-y := debug-monitors.o entry.o irq.o fpsimd.o \
return_address.o cpuinfo.o cpu_errata.o \
cpufeature.o alternative.o cacheinfo.o \
smp.o smp_spin_table.o topology.o smccc-call.o \
- syscall.o proton-pack.o idreg-override.o
+ syscall.o proton-pack.o idreg-override.o idle.o

targets += efi-entry.o

diff --git a/arch/arm64/kernel/idle.c b/arch/arm64/kernel/idle.c
new file mode 100644
index 000000000000..45c79204dc40
--- /dev/null
+++ b/arch/arm64/kernel/idle.c
@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Low-level idle sequences
+ */
+
+#include <linux/cpu.h>
+#include <linux/irqflags.h>
+
+#include <asm/arch_gicv3.h>
+#include <asm/barrier.h>
+#include <asm/cpufeature.h>
+#include <asm/sysreg.h>
+
+static void noinstr __cpu_do_idle(void)
+{
+ dsb(sy);
+ wfi();
+}
+
+static void noinstr __cpu_do_idle_irqprio(void)
+{
+ unsigned long pmr;
+ unsigned long daif_bits;
+
+ daif_bits = read_sysreg(daif);
+ write_sysreg(daif_bits | PSR_I_BIT | PSR_F_BIT, daif);
+
+ /*
+ * Unmask PMR before going idle to make sure interrupts can
+ * be raised.
+ */
+ pmr = gic_read_pmr();
+ gic_write_pmr(GIC_PRIO_IRQON | GIC_PRIO_PSR_I_SET);
+
+ __cpu_do_idle();
+
+ gic_write_pmr(pmr);
+ write_sysreg(daif_bits, daif);
+}
+
+/*
+ * cpu_do_idle()
+ *
+ * Idle the processor (wait for interrupt).
+ *
+ * If the CPU supports priority masking we must do additional work to
+ * ensure that interrupts are not masked at the PMR (because the core will
+ * not wake up if we block the wake up signal in the interrupt controller).
+ */
+void noinstr cpu_do_idle(void)
+{
+ if (system_uses_irq_prio_masking())
+ __cpu_do_idle_irqprio();
+ else
+ __cpu_do_idle();
+}
+
+/*
+ * This is our default idle handler.
+ */
+void noinstr arch_cpu_idle(void)
+{
+ /*
+ * This should do all the clock switching and wait for interrupt
+ * tricks
+ */
+ cpu_do_idle();
+ raw_local_irq_enable();
+}
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 2e7337709155..72c5d80f03fa 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -73,63 +73,6 @@ EXPORT_SYMBOL_GPL(pm_power_off);

void (*arm_pm_restart)(enum reboot_mode reboot_mode, const char *cmd);

-static void noinstr __cpu_do_idle(void)
-{
- dsb(sy);
- wfi();
-}
-
-static void noinstr __cpu_do_idle_irqprio(void)
-{
- unsigned long pmr;
- unsigned long daif_bits;
-
- daif_bits = read_sysreg(daif);
- write_sysreg(daif_bits | PSR_I_BIT | PSR_F_BIT, daif);
-
- /*
- * Unmask PMR before going idle to make sure interrupts can
- * be raised.
- */
- pmr = gic_read_pmr();
- gic_write_pmr(GIC_PRIO_IRQON | GIC_PRIO_PSR_I_SET);
-
- __cpu_do_idle();
-
- gic_write_pmr(pmr);
- write_sysreg(daif_bits, daif);
-}
-
-/*
- * cpu_do_idle()
- *
- * Idle the processor (wait for interrupt).
- *
- * If the CPU supports priority masking we must do additional work to
- * ensure that interrupts are not masked at the PMR (because the core will
- * not wake up if we block the wake up signal in the interrupt controller).
- */
-void noinstr cpu_do_idle(void)
-{
- if (system_uses_irq_prio_masking())
- __cpu_do_idle_irqprio();
- else
- __cpu_do_idle();
-}
-
-/*
- * This is our default idle handler.
- */
-void noinstr arch_cpu_idle(void)
-{
- /*
- * This should do all the clock switching and wait for interrupt
- * tricks
- */
- cpu_do_idle();
- raw_local_irq_enable();
-}
-
#ifdef CONFIG_HOTPLUG_CPU
void arch_cpu_idle_dead(void)
{