fix: gok vm: use cpu:host on macOS with Hypervisor.framework

[damdo]

macOS Hypervisor.framework (HVF) does not support named CPU models
like cortex-a72 in QEMU 10; only host and max are valid.

Use -cpu host when HVF acceleration is active and keep cortex-a72
for all other cases (Linux KVM, cross-arch software emulation).

ref: https://www.qemu.org/docs/master/system/arm/cpu-features.html

[damdo]

cc. @stapelberg

[damdo]

Closing and reopening to kick off testing again

[damdo]

I'm surprised others haven't seen this 😅
Maybe I'm the only one running nix unstable's qemu 😬
Have you noticed this @stapelberg ?

[stapelberg]

I'm surprised others haven't seen this 😅
Maybe I'm the only one running nix unstable's qemu 😬
Have you noticed this @stapelberg ?

I’m running qemu 10.2.2 from NixOS 26.05 (matches what’s in nixpkgs-unstable) and am not seeing this issue.

Are you running on an x86-64 host, too?

[stapelberg]

Unfortunately, -cpu=host does not work for me?

% qemu-system-aarch64 -name dr -boot order=d -drive file=/tmp/gokrazy-vm1161502338,format=raw -device i6300esb,id=watchdog0 -watchdog-action reset -smp 32 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp::8080-:80,hostfwd=tcp::8022-:22 -m 1024 -machine virt,highmem=off -cpu cortex-a72 -bios /tmp/gokrazy-vm1861489883/arm64-QEMU_EFI.fd
^Cqemu: terminating on signal 2

% qemu-system-aarch64 -name dr -boot order=d -drive file=/tmp/gokrazy-vm1161502338,format=raw -device i6300esb,id=watchdog0 -watchdog-action reset -smp 32 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp::8080-:80,hostfwd=tcp::8022-:22 -m 1024 -machine virt,highmem=off -cpu host -bios /tmp/gokrazy-vm1861489883/arm64-QEMU_EFI.fd      
qemu-system-aarch64: unable to find CPU model 'host'

[damdo]

Are you running on an x86-64 host, too?

Nope, running it on macOS's Darwin arm64

$ ./gok [...] vm run --graphic=false

...
2026/06/03 21:03:52
2026/06/03 21:03:52 running QEMU
[qemu-system-aarch64 -name gokrazy-dummy-arm64 -boot order=d -drive file=/var/folders/23/sd8wmjm93wl5895s_zk2kvfw0000gn/T/tmp.oNUBpyeOLw/gokrazy-vm1083429622,format=raw -device i6300esb,id=watchdog0 -watchdog-action reset -smp 12 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp::8080-:80,hostfwd=tcp::8022-:22 -m 1024 -machine virt,highmem=off -cpu cortex-a72 -bios /var/folders/23/sd8wmjm93wl5895s_zk2kvfw0000gn/T/tmp.oNUBpyeOLw/gokrazy-vm394458893/arm64-QEMU_EFI.fd -accel hvf -nographic]
qemu-system-aarch64: Invalid CPU model: cortex-a72
The valid models are: cortex-a53, cortex-a57, host, max
2026/06/03 21:03:53 [qemu-system-aarch64 -name gokrazy-dummy-arm64 -boot order=d -drive file=/var/folders/23/sd8wmjm93wl5895s_zk2kvfw0000gn/T/tmp.oNUBpyeOLw/gokrazy-vm1083429622,format=raw -device i6300esb,id=watchdog0 -watchdog-action reset -smp 12 -device e1000,netdev=net0 -netdev user,id=net0,hostfwd=tcp::8080-:80,hostfwd=tcp::8022-:22 -m 1024 -machine virt,highmem=off -cpu cortex-a72 -bios /var/folders/23/sd8wmjm93wl5895s_zk2kvfw0000gn/T/tmp.oNUBpyeOLw/gokrazy-vm394458893/arm64-QEMU_EFI.fd -accel hvf -nographic]: exit status 1
make: *** [vm] Error 1

$ ./gok version
https://github.com/gokrazy/tools/commit/e4396f9c2c7d

$ uname -a
Darwin mac 24.6.0 Darwin Kernel Version 24.6.0: Tue Apr 21 20:16:56 PDT 2026; root:xnu 11417.140.69.710.16~1/RELEASE_ARM64_T6030 arm64 arm Darwin

$ qemu-system-aarch64 --cpu help
Available CPUs:
  a64fx
  arm1026
  arm1136
  arm1136-r2
  arm1176
  arm11mpcore
  arm926
  arm946
  cortex-a15
  cortex-a35
  cortex-a53
  cortex-a55
  cortex-a57
  cortex-a7
  cortex-a710
  cortex-a72 <------
  cortex-a76
  cortex-a78ae
  cortex-a8
  cortex-a9
  cortex-m0
  cortex-m3
  cortex-m33
  cortex-m4
  cortex-m55
  cortex-m7
  cortex-r5
  cortex-r52
  cortex-r5f
  host
  max
  neoverse-n1
  neoverse-n2
  neoverse-v1
  sa1100
  sa1110
  ti925t

$ qemu-system-aarch64 -version
QEMU emulator version 10.2.1
Copyright (c) 2003-2025 Fabrice Bellard and the QEMU Project developers

The error is quite strange because -cpu help shows cortex-a72 to be there, but then it doesn't detect it 🤔

Even running it manually doesn't work

$ qemu-system-aarch64 -name gokrazy-dummy-arm64 -boot order=d -drive file=/var/folders/23/sd8wmjm93wl5895s_zk2kvfw0000gn/T/tmp.oNUBpyeOLw/gokrazy-vm1083429622,format=raw -device i6300esb,id=watchdog0 -watchdog-action reset -smp 12 -device e1000,netdev=net0 -netdev
 user,id=net0,hostfwd=tcp::8080-:80,hostfwd=tcp::8022-:22 -m 1024 -machine virt,highmem=off -cpu cortex-a72 -bios /var/folders/23/sd8wmjm93wl5895s_zk2kvfw0000gn/T/tmp.oNUBpyeOLw/gokrazy-vm394458893/arm64-QEMU_EFI.fd -accel hvf -nographic
qemu-system-aarch64: Invalid CPU model: cortex-a72
The valid models are: cortex-a53, cortex-a57, host, max

[damdo]

According to Gemini this might be a macOS only issue..

Why the Discrepancy?
--cpu help shows the Software Emulator (TCG) capabilities: When you ask QEMU for help, it lists every single ARM CPU architecture it is capable of emulating purely in software using its Tiny Code Generator (TCG). cortex-a72 is on that list because QEMU knows how to mimic it via software translation.
-accel hvf enforces Hardware Virtualization constraints: The moment gok appends -accel hvf, QEMU stops trying to emulate an ARM core in software. Instead, it hooks into macOS's native Hypervisor.framework (HVF) to run instructions directly on your Apple Silicon physical hardware.
Because you are passing instructions directly to your actual Mac CPU, you cannot pretend the core is a Raspberry Pi-style cortex-a72. The hypervisor strictly restricts the allowed -cpu targets to profiles compatible with your actual silicon. As your error message noted, under HVF, your choices drop down to: cortex-a53, cortex-a57, host, max.
Since gok automatically combines -accel hvf (because it sees you are on an arm64 Mac) with -cpu cortex-a72, it creates a command that QEMU rejects.

We could potentially use host on mac and keep cortex-a72 on linux

[damdo]

We could potentially use host on mac and keep cortex-a72 on linux

I've updated the PR in this direction. Hopefully that's reasonable. TY!

[stapelberg]

Thanks!