[PATCH v3 7/8] clocksource/drivers/arm_arch_timer: Expose a direct accessor for the virtual counter

[Marc Zyngier <maz@kernel.org>]

We allow access to the architected counter via arch_timer_read_counter().
However, this accessor can either be the virtual or the physical
view of the counter, depending on how the kernel has been booted.

At the same time, we have some architectural features (such as WFIT,
WFET) that rely on the virtual counter, and nothing else.

If implementations were perfect, we'd rely on reading CNTVCT_EL0,
and be done with it. However, we have a bunch of broken implementations
in the wild, which rely on preemption being disabled and other
costly workarounds.

In order to provide decent performance on non-broken HW while still
supporting the legacy horrors, expose arch_timer_read_vcounter() as
a new helper that hides this complexity. Obviously, this is simply
a global alias of arch_counter_get_cntvct().

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 drivers/clocksource/arm_arch_timer.c | 2 ++
 include/clocksource/arm_arch_timer.h | 1 +
 2 files changed, 3 insertions(+)

diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index b3b31d4f4815f..9c7501ce57073 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -128,6 +128,8 @@ static notrace u64 arch_counter_get_cntvct(void)
 	return __arch_counter_get_cntvct();
 }
 
+u64 arch_timer_read_vcounter(void) __attribute__((alias("arch_counter_get_cntvct")));
+
 /*
  * Default to cp15 based access because arm64 uses this function for
  * sched_clock() before DT is probed and the cp15 method is guaranteed
diff --git a/include/clocksource/arm_arch_timer.h b/include/clocksource/arm_arch_timer.h
index 8deb815ed0a08..32fd531ac009f 100644
--- a/include/clocksource/arm_arch_timer.h
+++ b/include/clocksource/arm_arch_timer.h
@@ -90,6 +90,7 @@ struct clock_read_data;
 
 extern u32 arch_timer_get_rate(void);
 extern u64 arch_timer_read_counter(void);
+extern u64 arch_timer_read_vcounter(void);
 extern struct arch_timer_kvm_info *arch_timer_get_kvm_info(void);
 extern bool arch_timer_evtstrm_available(void);
 extern bool read_sched_clock_is_arch_counter(const struct clock_read_data *);
-- 
2.47.3