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Documentation / virtual / kvm / arm / hyp-abi.txt


Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	* Internal ABI between the kernel and HYP
2	
3	This file documents the interaction between the Linux kernel and the
4	hypervisor layer when running Linux as a hypervisor (for example
5	KVM). It doesn't cover the interaction of the kernel with the
6	hypervisor when running as a guest (under Xen, KVM or any other
7	hypervisor), or any hypervisor-specific interaction when the kernel is
8	used as a host.
9	
10	On arm and arm64 (without VHE), the kernel doesn't run in hypervisor
11	mode, but still needs to interact with it, allowing a built-in
12	hypervisor to be either installed or torn down.
13	
14	In order to achieve this, the kernel must be booted at HYP (arm) or
15	EL2 (arm64), allowing it to install a set of stubs before dropping to
16	SVC/EL1. These stubs are accessible by using a 'hvc #0' instruction,
17	and only act on individual CPUs.
18	
19	Unless specified otherwise, any built-in hypervisor must implement
20	these 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	
46	Any other value of r0/x0 triggers a hypervisor-specific handling,
47	which is not documented here.
48	
49	The return value of a stub hypercall is held by r0/x0, and is 0 on
50	success, and HVC_STUB_ERR on error. A stub hypercall is allowed to
51	clobber any of the caller-saved registers (x0-x18 on arm64, r0-r3 and
52	ip on arm). It is thus recommended to use a function call to perform
53	the hypercall.
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