arm64: arch_timer: Allow an workaround descriptor to disable compat vdso

As we are about to disable the vdso for compat tasks in some circumstances,
let's allow a workaround descriptor to express exactly that.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200706163802.1836732-3-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/arch_timer.h

@@ -58,6 +58,7 @@ 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 *);
+	bool disable_compat_vdso;
 };
 
 DECLARE_PER_CPU(const struct arch_timer_erratum_workaround *,

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 if (wa->disable_compat_vdso && vdso_default != VDSO_CLOCKMODE_NONE) {
+		vdso_default = VDSO_CLOCKMODE_ARCHTIMER_NOCOMPAT;
+		clocksource_counter.vdso_clock_mode = vdso_default;
 	}
 }