[PATCH 4/5] clocksource/drivers/arm_arch_timer: Expose a direct accessor for the virtual counter

[Marc Zyngier]

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@xxxxxxxxxx>
---
 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 ee21804d6613c..6fcd9afad38c2 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -137,6 +137,8 @@ static notrace u64 arch_counter_get_cntvct(void)
 u64 (*arch_timer_read_counter)(void) __ro_after_init = arch_counter_get_cntvct;
 EXPORT_SYMBOL_GPL(arch_timer_read_counter);
 
+u64 arch_timer_read_vcounter(void) __attribute__((alias("arch_counter_get_cntvct")));
+
 static u64 arch_counter_read(struct clocksource *cs)
 {
 	return arch_timer_read_counter();
diff --git a/include/clocksource/arm_arch_timer.h b/include/clocksource/arm_arch_timer.h
index 2eda895f19f54..587314e584839 100644
--- a/include/clocksource/arm_arch_timer.h
+++ b/include/clocksource/arm_arch_timer.h
@@ -88,6 +88,7 @@ struct arch_timer_mem {
 
 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);
 
-- 
2.47.3