The Virtualization.framework does not provide an RTC device as /dev/rtc when a VM is booted using a Linux kernel image.

[norio-nomura]

Description

The Virtualization.framework does not provide an RTC device as /dev/rtc when a VM is booted using a Linux kernel image.

When booted from a disk image, rtc-efi is available as /dev/rtc0:

$ limactl create --name debug template://ubuntu-24.04 \
      --set '.mounts |= [{"location":"{{.Dir}}/log","mountPoint":"/var/log","writable": true}] + .' \
      --tty=false --log-level=warn
3.50 GiB / 3.50 GiB [---------------------------------------] 100.00% 1.63 GiB/s
$ limactl start debug --log-level=warn
$ head -1 ~/.lima/debug/log/kern.log 
2024-11-14T02:26:21.879392+00:00 lima-debug kernel: Booting Linux on physical CPU 0x0000000000 [0x610f0000]
$ grep rtc ~/.lima/debug/log/kern.log
2024-11-14T02:26:21.879850+00:00 lima-debug kernel: rtc-efi rtc-efi.0: registered as rtc0
2024-11-14T02:26:21.879850+00:00 lima-debug kernel: rtc-efi rtc-efi.0: setting system clock to 2024-11-14T02:26:16 UTC (1731551176)

However, when booting with a kernel image, no RTC devices (such as rtc-efi) are present, so the Linux kernel begins counting from the time configured at build:

$ hack/inject-cmdline-to-template.sh ~/.lima/debug/lima.yaml console=hvc0
kernel_location=https://cloud-images.ubuntu.com/releases/24.04/release-20241004/unpacked/ubuntu-24.04-server-cloudimg-arm64-vmlinuz-generic
kernel_digest=sha256:88e11de35134f64ca401ce11674b8480e416555255191b8dc95af69ec03cdd80
cmdline=root=LABEL=cloudimg-rootfs ro console=tty1 console=ttyAMA0 console=hvc0
initrd_location=https://cloud-images.ubuntu.com/releases/24.04/release-20241004/unpacked/ubuntu-24.04-server-cloudimg-arm64-initrd-generic
initrd_digest=sha256:5b380b89003c96bab04d43dc95ecb69eb25e46478fb3058efc6beea9ce5caf2b
$ limactl factory-reset debug --log-level=warn; rm -rf ~/.lima/debug/log
ERRO[0000] remove /Users/norio/.lima/debug/log: directory not empty 
$ limactl start debug --log-level=warn
WARN[0000] vmType vz: ignoring images[1]: [Kernel Initrd] 
17.39 MiB / 17.39 MiB [------------------------------------------] 100.00% ? p/s
3.50 GiB / 3.50 GiB [---------------------------------------] 100.00% 1.61 GiB/s
$ head -1 ~/.lima/debug/log/kern.log
2024-08-08T14:51:13.862963+00:00 lima-debug kernel: Booting Linux on physical CPU 0x0000000000 [0x610f0000]
$ grep rtc ~/.lima/debug/log/kern.log

Reference: https://developer.apple.com/forums/thread/760344

The system clock remains inaccurate until systemd-timesyncd.service synchronizes with an NTP server.

Known Issues Resulting from Lack of /dev/rtc0

1. Delayed NTP Synchronization: The time required for systemd-timesyncd.service to sync with an NTP server in the background can be lengthy—up to 30 seconds in some cases. If curl or apt-get install is executed in the foreground before synchronization completes, certificate validation fails, resulting in errors:

   [    6.489211] cloud-init[770]: + curl -fsSL https://get.docker.com
   [    6.558633] cloud-init[770]: curl: (60) SSL certificate problem: certificate is not yet valid
   [    6.558703] cloud-init[770]: More details here: https://curl.se/docs/sslcerts.html
   [    6.558761] cloud-init[770]: curl failed to verify the legitimacy of the server and therefore could not
   [    6.558834] cloud-init[770]: establish a secure connection to it. To learn more about this situation and
   [    6.558926] cloud-init[770]: how to fix it, please visit the web page mentioned above.
   [    6.559955] cloud-init[770]: LIMA 2024-08-08T23:51:16+09:00| WARNING: Failed to execute /mnt/lima-cidata/provision.system/00000002
   [    6.561851] cloud-init[770]: LIMA 2024-08-08T23:51:16+09:00| Executing /mnt/lima-cidata/provision.user/00000003 (as user norio)
   [�[0;32m  OK  �[0m] Started �[0;1;39msession-c1.scope�[0m - Session c1 of User norio.
   
   [    6.584176] cloud-init[770]: + command -v jq
   [    6.584344] cloud-init[770]: + sudo apt-get install --assume-yes jq
   [    6.723919] cloud-init[770]: Reading package lists...
   [    6.725890] cloud-init[770]: Building dependency tree...
   [    6.726179] cloud-init[770]: Reading state information...
   [    6.726716] cloud-init[770]: E: Unable to locate package jq
   [    6.728593] cloud-init[770]: LIMA 2024-08-08T23:51:16+09:00| WARNING: Failed to execute /mnt/lima-cidata/provision.user/00000003 (as user norio)

2. Guest Agent Time Synchronization Failure: fixSystemTimeSkew() in pkg/guestagent/guestagent_linux.go fails, preventing the guest OS from adjusting the time.

   $ limactl shell debug sudo journalctl -u lima-guestagent
   Nov 14 11:31:28 lima-debug systemd[1]: Started lima-guestagent.service - lima-guestagent.
   Nov 14 11:31:28 lima-debug systemd[1]: Stopping lima-guestagent.service - lima-guestagent...
   Nov 14 11:31:28 lima-debug systemd[1]: lima-guestagent.service: Deactivated successfully.
   Nov 14 11:31:28 lima-debug systemd[1]: Stopped lima-guestagent.service - lima-guestagent.
   Nov 14 11:31:28 lima-debug systemd[1]: Started lima-guestagent.service - lima-guestagent.
   Nov 14 11:31:28 lima-debug lima-guestagent[1727]: time="2024-11-14T11:31:28+09:00" level=info msg="event tick: 3s"
   Nov 14 11:31:28 lima-debug lima-guestagent[1727]: time="2024-11-14T11:31:28+09:00" level=info msg="serving the guest agent on vsock port: 2222"
   Nov 14 11:31:28 lima-debug lima-guestagent[1727]: time="2024-11-14T11:31:28+09:00" level=warning msg="fixSystemTimeSkew: error: stat /dev/rtc: no such file or directory"

Using a kernel image for booting allows specifying kernel command-line arguments, which has proven beneficial, e.g., in QEMU-based workarounds (#2541). Therefore, ensuring the system works as expected under these configurations is essential.

[norio-nomura]

I created PR #2894 to address the first issue mentioned above.