[PATCH 1/3] arm64: Fix sampling the "stable" virtual counter in preemptible section

[Marc Zyngier]

Ben reports that when running with CONFIG_DEBUG_PREEMPT, using
__arch_counter_get_cntvct_stable() results in well deserves warnings,
as we access a per-CPU variable without preemption disabled.

Fix the issue by disabling preemption on reading the counter. We can
probably do a lot better by not disabling preemption on systems that
do not require horrible workarounds to return a valid counter value,
but this plugs the issue for the time being.

Fixes: 29cc0f3aa7c64 ("arm64: Force the use of CNTVCT_EL0 in __delay()")
Reported-by: Ben Horgan <ben.horgan at arm.com>
Signed-off-by: Marc Zyngier <maz at kernel.org>
Link: https://lore.kernel.org/r/aZw3EGs4rbQvbAzV@e134344.arm.com
---
 arch/arm64/lib/delay.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/lib/delay.c b/arch/arm64/lib/delay.c
index d02341303899e..e278e060e78a9 100644
--- a/arch/arm64/lib/delay.c
+++ b/arch/arm64/lib/delay.c
@@ -32,7 +32,11 @@ static inline unsigned long xloops_to_cycles(unsigned long xloops)
  * Note that userspace cannot change the offset behind our back either,
  * as the vcpu mutex is held as long as KVM_RUN is in progress.
  */
-#define __delay_cycles()	__arch_counter_get_cntvct_stable()
+static cycles_t notrace __delay_cycles(void)
+{
+	guard(preempt_notrace)();
+	return __arch_counter_get_cntvct_stable();
+}
 
 void __delay(unsigned long cycles)
 {
-- 
2.47.3