hypervisor: add a generic guest-clock save/restore abstraction

Preserving the guest clock across pause/resume and snapshot/restore is
currently open-coded in the VMM against the x86-only
get_clock/set_clock. aarch64 needs the same correction but via a
different mechanism (i.e. the architected counter, CNTVCT). Having a
common backend-agnostic interface that VMM can drive uniformly allows us
to keep the architecture details behind the Hypervisor abstraction.

This commit only introduces the abstraction while the future commits
will actually move the implementation to use it.

Use this opportunity to fix the full path to get SystemTime as well.

Suggested-by: Sebastien Boeuf <sboeuf@meta.com>
Signed-off-by: Atish Patra <atishp@meta.com>

hypervisor/src/kvm/mod.rs

@@ -78,7 +78,7 @@ use x86_64::check_required_kvm_extensions;
 pub use x86_64::{CpuId, ExtendedControlRegisters, MsrEntries, VcpuKvmState};
 
 #[cfg(target_arch = "x86_64")]
-use crate::ClockData;
+use crate::{ClockData, ClockState};
 #[cfg(target_arch = "x86_64")]
 use crate::arch::x86::{
     CpuIdEntry, FpuState, LapicState, MTRR_MSR_INDICES, MsrEntry, NUM_IOAPIC_PINS,
@@ -1264,6 +1264,18 @@ impl vm::Vm for KvmVm {
             .map_err(|e| vm::HypervisorVmError::SetClock(e.into()))
     }
 
+    /// Capture kvmclock (filling realtime) for snapshot/migration.
+    #[cfg(target_arch = "x86_64")]
+    fn snapshot_clock(&self) -> vm::Result<Option<ClockState>> {
+        Ok(Some(self.get_clock()?.with_realtime_filled()))
+    }
+
+    /// Restore kvmclock before the vCPUs resume.
+    #[cfg(target_arch = "x86_64")]
+    fn restore_clock(&self, state: &ClockState) -> vm::Result<()> {
+        self.set_clock(state)
+    }
+
     /// Create a device that is used for passthrough
     fn create_passthrough_device(&self) -> vm::Result<VfioDeviceFd> {
         let mut vfio_dev = kvm_create_device {