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

Marc Zyngier
2020-07-02T13:52:29Z

On 2020-07-02 11:28, Mark Rutland wrote:
> 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;
> | }

Sure, that'd work too.

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

Not yet. Probably coming any time now. Which is why we may need to
look into per-CPU vdso data pages (Broonie has a series that could
be of some use, apparently).

Thanks,

M.
--
Jazz is not dead. It just smells funny...