performance: 0.910
hypervisor: 0.866
virtual: 0.855
device: 0.840
graphic: 0.777
semantic: 0.754
architecture: 0.661
user-level: 0.485
network: 0.459
debug: 0.448
mistranslation: 0.414
kernel: 0.385
permissions: 0.361
register: 0.346
vnc: 0.346
risc-v: 0.300
peripherals: 0.283
PID: 0.273
socket: 0.257
ppc: 0.244
i386: 0.220
x86: 0.219
boot: 0.218
arm: 0.208
files: 0.193
assembly: 0.163
TCG: 0.140
VMM: 0.131
KVM: 0.056

HVF virtual counter diverges from CLOCK_VIRTUAL when the host sleeps
Description of problem:
HVF's virtual counter diverges from `CLOCK_VIRTUAL` when the host sleeps and causes the inconsistency between Linux's system counter and everything else.

HVF's virtual counter apparently relies on something similar to `mach_absolute_time`, which stops when the host sleeps and resumes after it wakes up. However, `CLOCK_VIRTUAL` is implemented with `mach_continuous_time`, which continues even while the host sleeps. Linux uses the virtual counter as the source of the system counter and sees inconsistencies between the system counter and the other devices.
Steps to reproduce:
1. Launch Fedora.
2. Compare the time shown at the top of the guest display and one at the top of the host display. The difference should be less than 2 minutes.
3. Let the host sleep for 3 minutes.
4. Compare the times again. The difference is now greater than 2 minutes.
Additional information:
Here are solutions I've came up with so far. There are trade-offs but any of them should be better than the current situation. I'm happy to implement one if the maintainers have decided which one is the best or figure out a superior alternative.
- Implement `cpus_get_virtual_clock` of `AccelOpsClass` with `mach_absolute_time`. It would make HVF inconsistent with the other accelerators. Linux also expects the virtual clock is "continuous" and it leaves the divergence from the real time.
- Request XNU `HOST_NOTIFY_CALENDAR_CHANGE` to update the virtual clock with the continuous time. The interface is undocumented.
- Use `IORegisterForSystemPower` to update the virtual clock with the continuous time. It is undocumented that the interface handles every cases where `mach_absolute_time` and `mach_continuous_time`, but it actually does if I read XNU's source code correctly.