KVM: arm64: selftests: Add basic support to generate delays

Add udelay() support to generate a delay in the guest.

The routines are derived and simplified from kernel's
arch/arm64/lib/delay.c.
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211007233439.1826892-8-rananta@google.com

tools/testing/selftests/kvm/include/aarch64/delay.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * ARM simple delay routines
+ */
+
+#ifndef SELFTEST_KVM_ARM_DELAY_H
+#define SELFTEST_KVM_ARM_DELAY_H
+
+#include "arch_timer.h"
+
+static inline void __delay(uint64_t cycles)
+{
+	enum arch_timer timer = VIRTUAL;
+	uint64_t start = timer_get_cntct(timer);
+
+	while ((timer_get_cntct(timer) - start) < cycles)
+		cpu_relax();
+}
+
+static inline void udelay(unsigned long usec)
+{
+	__delay(usec_to_cycles(usec));
+}
+
+#endif /* SELFTEST_KVM_ARM_DELAY_H */