hypervisor: aarch64: capture the guest counter for snapshot/restore

Unlike x86, ARM64 has no kvmclock support to sync guest time upon
required. However, the guest reads the architected virtual timer
(CNTVCT_EL0) directly which can be modified by the VMM to update the
time after snapshot restore. Since the CNTVCT is in ticks, we also need
to read CNTFRQ (via mrs due to lack of ONEREG interface) to compute the
ticks from wall clock difference.

Because the counter is a vCPU register, the capture must run with the
vCPUs quiesced, so the VMM now captures the clock just after
cpu_manager.pause() through the boot vCPU. This is behaviorally
identical for x86, whose clock is VM-wide. There is no restore/advance
yet, so aarch64 guests still resume behind real time until the following
commit.

Signed-off-by: Atish Patra <atishp@meta.com>

hypervisor/src/arch/aarch64/mod.rs

@@ -7,6 +7,23 @@ pub mod regs;
 
 use serde::{Deserialize, Serialize};
 
+/// Reads the architected counter frequency (`CNTFRQ_EL0`, Hz) from the host: KVM
+/// does not expose it through ONE_REG and the guest counter runs at host frequency.
+pub fn get_cntfrq() -> u64 {
+    use std::arch::asm;
+    let cntfrq: u64;
+    // SAFETY: `mrs cntfrq_el0` only reads a read-only system register and
+    // touches no memory (nomem, nostack, preserves_flags).
+    unsafe {
+        asm!(
+            "mrs {}, cntfrq_el0",
+            out(reg) cntfrq,
+            options(nomem, nostack, preserves_flags),
+        );
+    }
+    cntfrq
+}
+
 #[derive(Clone, Serialize, Deserialize)]
 pub struct ExtendedReg {
     pub id: u64,