1* Internal ABI between the kernel and HYP 2 3This file documents the interaction between the Linux kernel and the 4hypervisor layer when running Linux as a hypervisor (for example 5KVM). It doesn't cover the interaction of the kernel with the 6hypervisor when running as a guest (under Xen, KVM or any other 7hypervisor), or any hypervisor-specific interaction when the kernel is 8used as a host. 9 10On arm and arm64 (without VHE), the kernel doesn't run in hypervisor 11mode, but still needs to interact with it, allowing a built-in 12hypervisor to be either installed or torn down. 13 14In order to achieve this, the kernel must be booted at HYP (arm) or 15EL2 (arm64), allowing it to install a set of stubs before dropping to 16SVC/EL1. These stubs are accessible by using a 'hvc #0' instruction, 17and only act on individual CPUs. 18 19Unless specified otherwise, any built-in hypervisor must implement 20these functions (see arch/arm{,64}/include/asm/virt.h): 21 22* r0/x0 = HVC_SET_VECTORS 23 r1/x1 = vectors 24 25 Set HVBAR/VBAR_EL2 to 'vectors' to enable a hypervisor. 'vectors' 26 must be a physical address, and respect the alignment requirements 27 of the architecture. Only implemented by the initial stubs, not by 28 Linux hypervisors. 29 30* r0/x0 = HVC_RESET_VECTORS 31 32 Turn HYP/EL2 MMU off, and reset HVBAR/VBAR_EL2 to the initials 33 stubs' exception vector value. This effectively disables an existing 34 hypervisor. 35 36* r0/x0 = HVC_SOFT_RESTART 37 r1/x1 = restart address 38 x2 = x0's value when entering the next payload (arm64) 39 x3 = x1's value when entering the next payload (arm64) 40 x4 = x2's value when entering the next payload (arm64) 41 42 Mask all exceptions, disable the MMU, move the arguments into place 43 (arm64 only), and jump to the restart address while at HYP/EL2. This 44 hypercall is not expected to return to its caller. 45 46Any other value of r0/x0 triggers a hypervisor-specific handling, 47which is not documented here. 48 49The return value of a stub hypercall is held by r0/x0, and is 0 on 50success, and HVC_STUB_ERR on error. A stub hypercall is allowed to 51clobber any of the caller-saved registers (x0-x18 on arm64, r0-r3 and 52ip on arm). It is thus recommended to use a function call to perform 53the hypercall. 54