x86_64: lock IA32_FEATURE_CONTROL with VMXON enabled at boot#438
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Guest hypervisors (Windows Hyper-V, WSL2) check IA32_FEATURE_CONTROL at boot and refuse VMXON unless firmware has set the lock bit with VMXON-outside-SMX enabled -- otherwise they report "virtualization not enabled in firmware". rust-hypervisor-firmware never touched this MSR, so a Windows guest saw VMX in CPUID (VMMonitorModeExtensions=True) but VirtualizationFirmwareEnabled=False, and Hyper-V launch failed with Event 41 "Either VMX not present or not enabled in BIOS". Nested virtualization was therefore unusable. Set the MSR the way real firmware does: when CPUID reports VMX, lock IA32_FEATURE_CONTROL with VMXON-outside-SMX enabled, before handing control to the OS. Skipped when the lock bit is already set (idempotent) and when VMX is absent (AMD / nested off), so AMD and non-nested guests are unaffected. Verified on a Cloud Hypervisor Windows 11 guest: VirtualizationFirmwareEnabled flips False to True and the guest boots normally. Signed-off-by: tonic <tonicbupt@gmail.com>
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Guest hypervisors (Windows Hyper-V, WSL2) check
IA32_FEATURE_CONTROL(MSR0x3A) at boot and refuseVMXONunless the firmware has set the lock bitwith VMXON-outside-SMX enabled — otherwise they report "virtualization not
enabled in firmware". rust-hypervisor-firmware never touched this MSR, so a
Windows guest saw VMX in CPUID (
VMMonitorModeExtensions=True) butVirtualizationFirmwareEnabled=False, and Hyper-V launch failed with Event 41"Either VMX not present or not enabled in BIOS". Nested virtualization was
therefore unusable.
Set the MSR the way real firmware does: in
rust64_start, whenCPUID.1:ECX.VMXreports VMX support, lock
IA32_FEATURE_CONTROLwith bit 0 (LOCKED) and bit 2(VMXON-outside-SMX), before handing control to the OS. The write is skipped when
the lock bit is already set (idempotent) and when VMX is absent (AMD / nested
off), so AMD and non-nested guests are unaffected. Other architectures
(aarch64, riscv64) are not touched.
Verified on a Cloud Hypervisor Windows 11 guest:
VirtualizationFirmwareEnabledflips False to True and the guest boots normally.
Signed-off-by: tonic tonicbupt@gmail.com