arm64: idle: don't instrument idle code with KCOV

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>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210607094624.34689-21-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/kernel/Makefile

@@ -18,6 +18,7 @@ CFLAGS_syscall.o	+= -fno-stack-protector
 # available or are out-of-sync with HW state. Since `noinstr` doesn't always
 # inhibit KCOV instrumentation, disable it for the entire compilation unit.
 KCOV_INSTRUMENT_entry.o := n
+KCOV_INSTRUMENT_idle.o := n
 
 # Object file lists.
 obj-y			:= debug-monitors.o entry.o irq.o fpsimd.o		\
@@ -27,7 +28,7 @@ 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
 

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();
+}

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)
 {