Re: [PATCH 2/3] arm64: arch_timer: Allow an workaround descriptor to provide vdso_clock_mode

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Jul 01, 2020 at 05:18:23PM +0100, Marc Zyngier wrote:
> As we are about to disable the vdso for compat tasks in some circumstances,
> let's allow a workaround descriptor to provide the vdso_clock_mode that
> matches the platform.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/include/asm/arch_timer.h  | 3 +++
>  drivers/clocksource/arm_arch_timer.c | 3 +++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> index 7ae54d7d333a..fb8dfcbf9c01 100644
> --- a/arch/arm64/include/asm/arch_timer.h
> +++ b/arch/arm64/include/asm/arch_timer.h
> @@ -18,6 +18,8 @@
>  #include <linux/smp.h>
>  #include <linux/types.h>
>  
> +#include <vdso/clocksource.h>
> +
>  #include <clocksource/arm_arch_timer.h>
>  
>  #if IS_ENABLED(CONFIG_ARM_ARCH_TIMER_OOL_WORKAROUND)
> @@ -58,6 +60,7 @@ struct arch_timer_erratum_workaround {
>  	u64 (*read_cntvct_el0)(void);
>  	int (*set_next_event_phys)(unsigned long, struct clock_event_device *);
>  	int (*set_next_event_virt)(unsigned long, struct clock_event_device *);
> +	enum vdso_clock_mode vdso_clock_mode;
>  };
>  
>  DECLARE_PER_CPU(const struct arch_timer_erratum_workaround *,
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index ecf7b7db2d05..f828835c568f 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -566,6 +566,9 @@ void arch_timer_enable_workaround(const struct arch_timer_erratum_workaround *wa
>  	if (wa->read_cntvct_el0) {
>  		clocksource_counter.vdso_clock_mode = VDSO_CLOCKMODE_NONE;
>  		vdso_default = VDSO_CLOCKMODE_NONE;
> +	} else {
> +		clocksource_counter.vdso_clock_mode = wa->vdso_clock_mode;
> +		vdso_default = wa->vdso_clock_mode;
>  	}

I fear that we're liable to forget to set vdso_clock_mode on new errata
that don't need a read_cntvct_el0 hook, and if so we'll happen to set
these to 0 (i.e. VDSO_CLOCKMODE_NONE).

For now could we instead have a boolan disable_compat_vdso, with this
being:

| if (wa->read_cntvct_el0) {
| 	clocksource_counter.vdso_clock_mode = VDSO_CLOCKMODE_NONE;
| 	vdso_default = VDSO_CLOCKMODE_NONE;
| } else if (wa->disable_compat_vdso) {
| 	clocksource_counter.vdso_clock_mode = VDSO_CLOCKMODE_ARCHTIMER_NOCOMPAT;
| 	vdso_default = wa->vdso_clock_mode = VDSO_CLOCKMODE_ARCHTIMER_NOCOMPAT;
| }

Do we need to handle the comination of a workaround seeting NONE and one
setting ARCHTIMER_NOCOMPAT?

Mark.