Internal ABI between the kernel and HYPΒΆ

This file documents the interaction between the Linux kernel and the hypervisor layer when running Linux as a hypervisor (for example KVM). It doesn't cover the interaction of the kernel with the hypervisor when running as a guest (under Xen, KVM or any other hypervisor), or any hypervisor-specific interaction when the kernel is used as a host.

On arm and arm64 (without VHE), the kernel doesn't run in hypervisor mode, but still needs to interact with it, allowing a built-in hypervisor to be either installed or torn down.

In order to achieve this, the kernel must be booted at HYP (arm) or EL2 (arm64), allowing it to install a set of stubs before dropping to SVC/EL1. These stubs are accessible by using a 'hvc #0' instruction, and only act on individual CPUs.

Unless specified otherwise, any built-in hypervisor must implement these functions (see arch/arm{,64}/include/asm/virt.h):

  • r0/x0 = HVC_SET_VECTORS
    r1/x1 = vectors
    

    Set HVBAR/VBAR_EL2 to 'vectors' to enable a hypervisor. 'vectors' must be a physical address, and respect the alignment requirements of the architecture. Only implemented by the initial stubs, not by Linux hypervisors.

  • r0/x0 = HVC_RESET_VECTORS
    

    Turn HYP/EL2 MMU off, and reset HVBAR/VBAR_EL2 to the initials stubs' exception vector value. This effectively disables an existing hypervisor.

  • r0/x0 = HVC_SOFT_RESTART
    r1/x1 = restart address
    x2 = x0's value when entering the next payload (arm64)
    x3 = x1's value when entering the next payload (arm64)
    x4 = x2's value when entering the next payload (arm64)
    

    Mask all exceptions, disable the MMU, move the arguments into place (arm64 only), and jump to the restart address while at HYP/EL2. This hypercall is not expected to return to its caller.

Any other value of r0/x0 triggers a hypervisor-specific handling, which is not documented here.

The return value of a stub hypercall is held by r0/x0, and is 0 on success, and HVC_STUB_ERR on error. A stub hypercall is allowed to clobber any of the caller-saved registers (x0-x18 on arm64, r0-r3 and ip on arm). It is thus recommended to use a function call to perform the hypercall.