1.. SPDX-License-Identifier: GPL-2.0
2
3=======================================
4Internal ABI between the kernel and HYP
5=======================================
6
7This file documents the interaction between the Linux kernel and the
8hypervisor layer when running Linux as a hypervisor (for example
9KVM). It doesn't cover the interaction of the kernel with the
10hypervisor when running as a guest (under Xen, KVM or any other
11hypervisor), or any hypervisor-specific interaction when the kernel is
12used as a host.
13
14Note: KVM/arm has been removed from the kernel. The API described
15here is still valid though, as it allows the kernel to kexec when
16booted at HYP. It can also be used by a hypervisor other than KVM
17if necessary.
18
19On arm and arm64 (without VHE), the kernel doesn't run in hypervisor
20mode, but still needs to interact with it, allowing a built-in
21hypervisor to be either installed or torn down.
22
23In order to achieve this, the kernel must be booted at HYP (arm) or
24EL2 (arm64), allowing it to install a set of stubs before dropping to
25SVC/EL1. These stubs are accessible by using a 'hvc #0' instruction,
26and only act on individual CPUs.
27
28Unless specified otherwise, any built-in hypervisor must implement
29these functions (see arch/arm{,64}/include/asm/virt.h):
30
31* ::
32
33    r0/x0 = HVC_SET_VECTORS
34    r1/x1 = vectors
35
36  Set HVBAR/VBAR_EL2 to 'vectors' to enable a hypervisor. 'vectors'
37  must be a physical address, and respect the alignment requirements
38  of the architecture. Only implemented by the initial stubs, not by
39  Linux hypervisors.
40
41* ::
42
43    r0/x0 = HVC_RESET_VECTORS
44
45  Turn HYP/EL2 MMU off, and reset HVBAR/VBAR_EL2 to the initials
46  stubs' exception vector value. This effectively disables an existing
47  hypervisor.
48
49* ::
50
51    r0/x0 = HVC_SOFT_RESTART
52    r1/x1 = restart address
53    x2 = x0's value when entering the next payload (arm64)
54    x3 = x1's value when entering the next payload (arm64)
55    x4 = x2's value when entering the next payload (arm64)
56
57  Mask all exceptions, disable the MMU, clear I+D bits, move the arguments
58  into place (arm64 only), and jump to the restart address while at HYP/EL2.
59  This hypercall is not expected to return to its caller.
60
61* ::
62
63    x0 = HVC_VHE_RESTART (arm64 only)
64
65  Attempt to upgrade the kernel's exception level from EL1 to EL2 by enabling
66  the VHE mode. This is conditioned by the CPU supporting VHE, the EL2 MMU
67  being off, and VHE not being disabled by any other means (command line
68  option, for example).
69
70Any other value of r0/x0 triggers a hypervisor-specific handling,
71which is not documented here.
72
73The return value of a stub hypercall is held by r0/x0, and is 0 on
74success, and HVC_STUB_ERR on error. A stub hypercall is allowed to
75clobber any of the caller-saved registers (x0-x18 on arm64, r0-r3 and
76ip on arm). It is thus recommended to use a function call to perform
77the hypercall.
78