Re: [PATCH 3/9] intel_idle: Relocate definitions of cpuidle callbacks

Qian Cai
2020-05-23T00:04:33Z

On Thu, Feb 13, 2020 at 11:00:26PM +0100, Rafael J. Wysocki wrote:
> From: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
>
> Move the definitions of intel_idle() and intel_idle_s2idle() before
> the definitions of cpuidle_state structures referring to them to
> avoid having to use additional declarations of them (and drop those
> declarations).
>
> No functional impact.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> drivers/idle/intel_idle.c | 154 ++++++++++++++++++++++------------------------
> 1 file changed, 75 insertions(+), 79 deletions(-)
>
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index 5adc058c705d..e0332d567735 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
[]
> +/**
> + * intel_idle - Ask the processor to enter the given idle state.
> + * @dev: cpuidle device of the target CPU.
> + * @drv: cpuidle driver (assumed to point to intel_idle_driver).
> + * @index: Target idle state index.
> + *
> + * Use the MWAIT instruction to notify the processor that the CPU represented by
> + * @dev is idle and it can try to enter the idle state corresponding to @index.
> + *
> + * If the local APIC timer is not known to be reliable in the target idle state,
> + * enable one-shot tick broadcasting for the target CPU before executing MWAIT.
> + *
> + * Optionally call leave_mm() for the target CPU upfront to avoid wakeups due to
> + * flushing user TLBs.
> + *
> + * Must be called under local_irq_disable().
> + */
> +static __cpuidle int intel_idle(struct cpuidle_device *dev,
> + struct cpuidle_driver *drv, int index)
> +{
> + struct cpuidle_state *state = &drv->states[index];
> + unsigned long eax = flg2MWAIT(state->flags);
> + unsigned long ecx = 1; /* break on interrupt flag */
> + bool uninitialized_var(tick);

This will generate an UBSAN warning because Clang could poison all
uninitialized stack variables to 0xAA due to CONFIG_INIT_STACK_ALL=y, so one
issue is that,

bool uninitialized_var(x);

would always broken on Clang like this,

[ 92.140611] UBSAN: invalid-load in drivers/idle/intel_idle.c:135:7
[ 92.143111] load of value 170 is not a valid value for type 'bool' (aka '_Bool')
[ 92.145657] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.7.0-rc6-next-20200522+ #3
[ 92.147424] Hardware name: HP ProLiant BL660c Gen9, BIOS I38 10/17/2018
[ 92.149869] Call Trace:
[ 92.149869] dump_stack+0x10b/0x17f
[ 92.149869] __ubsan_handle_load_invalid_value+0xd2/0x110
[ 92.149869] intel_idle+0x54/0xf0
[ 92.156202] cpuidle_enter_state+0x120/0x4f0
[ 92.156202] cpuidle_enter+0x5b/0xa0
[ 92.156202] call_cpuidle+0x25/0x50
[ 92.156202] do_idle+0x1eb/0x2c0
[ 92.156202] cpu_startup_entry+0x25/0x30
[ 92.156202] rest_init+0x26f/0x280
[ 92.156202] arch_call_rest_init+0x17/0x1e
[ 92.156202] start_kernel+0x598/0x633
[ 92.156202] x86_64_start_reservations+0x24/0x26
[ 92.156202] x86_64_start_kernel+0x116/0x1c1
[ 92.156202] secondary_startup_64+0xb6/0xc0

However, I am wondering if it is correct to let "tick" uninitialized to begin
with. If this condition is true,

!static_cpu_has(X86_FEATURE_ARAT) && lapic_timer_always_reliable

Then, we could in the final branch to use the uninitialized value.

if (!static_cpu_has(X86_FEATURE_ARAT) && tick)

Isn't that possible?

> + int cpu = smp_processor_id();
> +
> + /*
> + * leave_mm() to avoid costly and often unnecessary wakeups
> + * for flushing the user TLB's associated with the active mm.
> + */
> + if (state->flags & CPUIDLE_FLAG_TLB_FLUSHED)
> + leave_mm(cpu);
> +
> + if (!static_cpu_has(X86_FEATURE_ARAT) && !lapic_timer_always_reliable) {
> + /*
> + * Switch over to one-shot tick broadcast if the target C-state
> + * is deeper than C1.
> + */
> + if ((eax >> MWAIT_SUBSTATE_SIZE) & MWAIT_CSTATE_MASK) {
> + tick = true;
> + tick_broadcast_enter();
> + } else {
> + tick = false;
> + }
> + }
> +
> + mwait_idle_with_hints(eax, ecx);
> +
> + if (!static_cpu_has(X86_FEATURE_ARAT) && tick)
> + tick_broadcast_exit();
> +
> + return index;
> +}