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-id = 864
-title = "HVF virtual counter diverges from CLOCK_VIRTUAL when the host sleeps"
-state = "opened"
-created_at = "2022-02-10T01:36:27.240Z"
-closed_at = "n/a"
-labels = ["accel: HVF", "target: arm"]
-url = "https://gitlab.com/qemu-project/qemu/-/issues/864"
-host-os = "macOS"
-host-arch = "arm64"
-qemu-version = "6621441db50d5bae7e34dbd04bf3c57a27a71b32"
-guest-os = "Fedora 35"
-guest-arch = "aarch64"
-description = """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."""
-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 = """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."""