Re: [Minios-devel] [UNIKRAFT PATCHv5 26/46] plat/common: Add counter workaround for Cortex-A73 erratum 858921

[Julien Grall <julien.grall@xxxxxxx>]

Hi Wei,

On 10/08/18 08:08, Wei Chen wrote:
> The errata #858921 describes that Cortex-A73 (r0p0 - r0p2)
> counter read can return a wrong value when the counter crosses
> a 32bit boundary, but newer Cortex-A73 are not affected.
>
> The workaround involves performing the read twice, compare
> bit[32] of the two read values. If bit[32] is different,
> keep the first value, otherwise keep the second value.
>
> Signed-off-by: Wei Chen <wei.chen@xxxxxxx>
> ---
>   arch/arm/arm64/Config.uk |  9 +++++++++
>   plat/common/arm/time.c   | 20 ++++++++++++++++++++
>   2 files changed, 29 insertions(+)
>
> diff --git a/arch/arm/arm64/Config.uk b/arch/arm/arm64/Config.uk
> index 7797516..07bc8ec 100644
> --- a/arch/arm/arm64/Config.uk
> +++ b/arch/arm/arm64/Config.uk
> @@ -46,3 +46,12 @@ config MARCH_ARM64_CORTEXA75
>                 Compile for Armv8.2 Cortex-A75 (and compatible) CPUs
>   
>   endchoice
> +
> +config ARM64_ERRATUM_858921
> +       bool "Workaround for Cortex-A73 erratum 858921"
> +       default n
> +       depends on MARCH_ARM64_CORTEXA73
I don't think this is correct here. MARCH_ARM64_CORTEXA73 is about how 
the code was optimized for a given processor. It would still be possible 
to run a code compiled with generic option on Cortex-A73.
So you want to at least drop the depends on here and possibly default y 
for CORTEX_A73 and GENERIC.
Cheers,


> +       help
> +         This option enables a workaround for Cortex-A73 (r0p0 - r0p2),
> +         whose counter may return a wrong value when the counter crosses
> +         a 32-bit boundary. The newer Cortex-A73 are not affected.
> diff --git a/plat/common/arm/time.c b/plat/common/arm/time.c
> index 4d955f3..1829938 100644
> --- a/plat/common/arm/time.c
> +++ b/plat/common/arm/time.c
> @@ -60,10 +60,30 @@ static inline uint64_t get_counter_frequency(void)
>         return SYSREG_READ(cntfrq_el0);
>   }
>   
> +#ifdef CONFIG_ARM64_ERRATUM_858921
> +/*
> + * The errata #858921 describes that Cortex-A73 (r0p0 - r0p2) counter
> + * read can return a wrong value when the counter crosses a 32bit boundary.
> + * But newer Cortex-A73 are not affected.
> + *
> + * The workaround involves performing the read twice, compare bit[32] of
> + * the two read values. If bit[32] is different, keep the first value,
> + * otherwise keep the second value.
> + */
> +static uint64_t read_virtual_count(void)
> +{
> +    uint64_t val_1st, val_2nd;
> +
> +    val_1st = SYSREG_READ(cntvct_el0);
> +    val_2nd = SYSREG_READ(cntvct_el0);
> +    return (((val_1st ^ val_2nd) >> 32) & 1) ? val_1st : val_2nd;
> +}
> +#else
>   static inline uint64_t read_virtual_count(void)
>   {
>         return SYSREG_READ(cntvct_el0);
>   }
> +#endif
>   
>   /*
>    * monotonic_clock(): returns # of nanoseconds passed since
--
Julien Grall

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