Re: ERRATUM_858921 is broken on 5.15 kernel

[Yogesh Lal <quic_ylal@quicinc.com>]

On 1/5/2023 7:52 PM, Marc Zyngier wrote:
> On Thu, 05 Jan 2023 13:33:48 +0000,
> Yogesh Lal <quic_ylal@quicinc.com> wrote:
>> Hi,
>>
>> We are observing issue on A73 core where ERRATUM_858921 is broken.
>>
>> On 5.15 kernel arch_timer_enable_workaround is set by reading
>> arm64_858921_read_cntpct_el0 and arm64_858921_read_cntvct_el0 during
>> timer register using following path.
> Have you checked whether the issue is still present on 6.1?
yes, its preset there as well.
>
>> arch_timer_enable_workaround->atomic_set(&timer_unstable_counter_workaround_in_use,
>> 1);
>>
>> [code snap]
>> 564 static
>> 565 void arch_timer_enable_workaround(const struct
>> arch_timer_erratum_workaround *wa,
>> 566                               bool local)
>> 567 {
>> 568     int i;
>> 569
>> 570     if (local) {
>> 571 __this_cpu_write(timer_unstable_counter_workaround, wa);
>> 572     } else {
>> 573             for_each_possible_cpu(i)
>> 574                     per_cpu(timer_unstable_counter_workaround, i) = wa;
>> 575     }
>> 576
>> 577     if (wa->read_cntvct_el0 || wa->read_cntpct_el0)
>> 578 atomic_set(&timer_unstable_counter_workaround_in_use, 1);
>>
>>
>> and based on above workaround enablement , appropriate function to get
>> counter is used.
>>
>> 1008 static void __init arch_counter_register(unsigned type)
>> 1009 {
>> 1010     u64 start_count;
>> 1011
>> 1012     /* Register the CP15 based counter if we have one */
>> 1013     if (type & ARCH_TIMER_TYPE_CP15) {
>> 1014         u64 (*rd)(void);
>> 1015
>> 1016         if ((IS_ENABLED(CONFIG_ARM64) && !is_hyp_mode_available()) ||
>> 1017             arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI) {
>> 1018             if (arch_timer_counter_has_wa())
>> 1019                 rd = arch_counter_get_cntvct_stable;
>> 1020             else
>> 1021                 rd = arch_counter_get_cntvct;
>> 1022         } else {
>> 1023             if (arch_timer_counter_has_wa())
>> 1024                 rd = arch_counter_get_cntpct_stable;
>> 1025             else
>> 1026                 rd = arch_counter_get_cntpct;
>> 1027         }
>> [snap]
>> 1043     /* 56 bits minimum, so we assume worst case rollover */
>> 1044     sched_clock_register(arch_timer_read_counter, 56, arch_timer_rate);
>>
>>
>> As our boot cores are not impacted by errata sched_clock_register()
>> will register !arch_timer_counter_has_wa() callback.
>>
>> Now when errata impacted core boots up and sched_clock_register
>> already register will !arch_timer_counter_has_wa() path.
>> As sched_clock_register is not per_cpu bases so
>> arch_timer_read_counter will always point to
>> !arch_timer_counter_has_wa() function calls.
> Please try the following hack, only compile tested as I do not have
> access to any affected HW, and report whether this solves your issue
> or not. Note that this is based on 6.2-rc2.

tested it on affected h/w but looks like sched_clock is still pointing 
to !arch_timer_counter_has_wa() function calls,
may be due to sched_clock_register will register once during non errata 
impacted core booting.

1007 static void __init arch_counter_register(unsigned type)
1008 {
[snap]
1043     /* 56 bits minimum, so we assume worst case rollover */
1044     sched_clock_register(arch_timer_read_counter, 56, arch_timer_rate);


Also update_clock_read_data will called with 
!arch_timer_counter_has_wa() read function calls.

153 void sched_clock_register(u64 (*read)(void), int bits, unsigned long 
rate)
[snap]
183     cd.actual_read_sched_clock = read;
184
185     rd.read_sched_clock    = read;
186     rd.sched_clock_mask    = new_mask;
187     rd.mult            = new_mult;
188     rd.shift        = new_shift;
189     rd.epoch_cyc        = new_epoch;
190     rd.epoch_ns        = ns;
191
192     update_clock_read_data(&rd);

> Thanks,
>
> 	M.
>
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index e09d4427f604..a7cf0a2c86d3 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -230,6 +230,28 @@ static u64 arch_counter_read_cc(const struct cyclecounter *cc)
>   	return arch_timer_read_counter();
>   }
>   
> +static bool arch_timer_counter_has_wa(void);
> +
> +static u64 (*arch_counter_get_read_fn(void))(void)
> +{
> +	u64 (*rd)(void);
> +
> +	if ((IS_ENABLED(CONFIG_ARM64) && !is_hyp_mode_available()) ||
> +	    arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI) {
> +		if (arch_timer_counter_has_wa())
> +			rd = arch_counter_get_cntvct_stable;
> +		else
> +			rd = arch_counter_get_cntvct;
> +	} else {
> +		if (arch_timer_counter_has_wa())
> +			rd = arch_counter_get_cntpct_stable;
> +		else
> +			rd = arch_counter_get_cntpct;
> +	}
> +
> +	return rd;
> +}
> +
>   static struct clocksource clocksource_counter = {
>   	.name	= "arch_sys_counter",
>   	.id	= CSID_ARM_ARCH_COUNTER,
> @@ -571,8 +593,10 @@ void arch_timer_enable_workaround(const struct arch_timer_erratum_workaround *wa
>   			per_cpu(timer_unstable_counter_workaround, i) = wa;
>   	}
>   
> -	if (wa->read_cntvct_el0 || wa->read_cntpct_el0)
> +	if (wa->read_cntvct_el0 || wa->read_cntpct_el0) {
>   		atomic_set(&timer_unstable_counter_workaround_in_use, 1);
> +		arch_timer_read_counter = arch_counter_get_read_fn();
> +	}
>   
>   	/*
>   	 * Don't use the vdso fastpath if errata require using the
> @@ -641,7 +665,7 @@ static bool arch_timer_counter_has_wa(void)
>   #else
>   #define arch_timer_check_ool_workaround(t,a)		do { } while(0)
>   #define arch_timer_this_cpu_has_cntvct_wa()		({false;})
> -#define arch_timer_counter_has_wa()			({false;})
> +static bool arch_timer_counter_has_wa(void)		{ return false; }
>   #endif /* CONFIG_ARM_ARCH_TIMER_OOL_WORKAROUND */
>   
>   static __always_inline irqreturn_t timer_handler(const int access,
> @@ -1079,22 +1103,7 @@ static void __init arch_counter_register(unsigned type)
>   
>   	/* Register the CP15 based counter if we have one */
>   	if (type & ARCH_TIMER_TYPE_CP15) {
> -		u64 (*rd)(void);
> -
> -		if ((IS_ENABLED(CONFIG_ARM64) && !is_hyp_mode_available()) ||
> -		    arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI) {
> -			if (arch_timer_counter_has_wa())
> -				rd = arch_counter_get_cntvct_stable;
> -			else
> -				rd = arch_counter_get_cntvct;
> -		} else {
> -			if (arch_timer_counter_has_wa())
> -				rd = arch_counter_get_cntpct_stable;
> -			else
> -				rd = arch_counter_get_cntpct;
> -		}
> -
> -		arch_timer_read_counter = rd;
> +		arch_timer_read_counter = arch_counter_get_read_fn();
>   		clocksource_counter.vdso_clock_mode = vdso_default;
>   	} else {
>   		arch_timer_read_counter = arch_counter_get_cntvct_mem;
>