sys/cpu: Use cntvct_el0 instead of cntpct_el0 on aarch64 - #712

Summary

• Switched AArch64 timer counter reads from the physical counter (CNTPCT_EL0) to the virtual counter (CNTVCT_EL0) in common/sys/cpu.h and checks/payloads/limine/arch_aarch64.c.
• Eliminated EL1 trap overhead and synchronous exceptions when booting Limine or running the AArch64 test payload under hypervisors or firmware configurations that restrict physical counter access.

Background & Problem

Per the ARM Architecture Reference Manual for A-profile architecture (ARM DDI 0487), CNTPCT_EL0 reads the physical counter, whereas CNTVCT_EL0 reads the virtual counter, i.e. the physical count minus CNTVOFF_EL2 (treated as zero when EL2 is absent or disabled). Access from lower exception levels may be trapped by the hypervisor.

1. Hypervisor traps and EL1 access restrictions: Under EL2 hypervisors (such as KVM/QEMU) or firmware environments where CNTHCTL_EL2.EL1PCTEN is cleared, accessing CNTPCT_EL0 from EL1 traps to EL2. Depending on hypervisor configuration, this either incurs costly VM-exit emulation overhead on every timer poll or triggers an unhandled synchronous exception.
2. Standard EL1 timer interface: CNTVCT_EL0 is intended for EL1/EL0 consumers, executes natively in hardware without trapping to EL2 under standard virtualisation setups, and provides a consistent monotonic counter on both bare metal and virtualised guests.

Changes

1. common/sys/cpu.h: Updated rdtsc() on AArch64 to read cntvct_el0 instead of cntpct_el0.
2. checks/payloads/limine/arch_aarch64.c: Updated arch_counter() in the Limine test payload to read cntvct_el0 instead of cntpct_el0.

Verification & Testing

1. Full Build:
   • Clean compilation under -Werror with 0 warnings and 0 errors.
2. Test Suite:
   • Executed the AArch64 UEFI test suite in QEMU, confirming that both the bootloader timer routines and the test payload counter read CNTVCT_EL0 cleanly without exceptions.