1.. SPDX-License-Identifier: GPL-2.0
2
3===================================
4The LoongArch paravirtual interface
5===================================
6
7KVM hypercalls use the HVCL instruction with code 0x100 and the hypercall
8number is put in a0. Up to five arguments may be placed in registers a1 - a5.
9The return value is placed in v0 (an alias of a0).
10
11Source code for this interface can be found in arch/loongarch/kvm*.
12
13Querying for existence
14======================
15
16To determine if the host is running on KVM, we can utilize the cpucfg()
17function at index CPUCFG_KVM_BASE (0x40000000).
18
19The CPUCFG_KVM_BASE range, spanning from 0x40000000 to 0x400000FF, The
20CPUCFG_KVM_BASE range between 0x40000000 - 0x400000FF is marked as reserved.
21Consequently, all current and future processors will not implement any
22feature within this range.
23
24On a KVM-virtualized Linux system, a read operation on cpucfg() at index
25CPUCFG_KVM_BASE (0x40000000) returns the magic string 'KVM\0'.
26
27Once you have determined that your host is running on a paravirtualization-
28capable KVM, you may now use hypercalls as described below.
29
30KVM hypercall ABI
31=================
32
33The KVM hypercall ABI is simple, with one scratch register a0 (v0) and at most
34five generic registers (a1 - a5) used as input parameters. The FP (Floating-
35point) and vector registers are not utilized as input registers and must
36remain unmodified during a hypercall.
37
38Hypercall functions can be inlined as it only uses one scratch register.
39
40The parameters are as follows:
41
42	========	=================	================
43	Register	IN			OUT
44	========	=================	================
45	a0		function number		Return	code
46	a1		1st	parameter	-
47	a2		2nd	parameter	-
48	a3		3rd	parameter	-
49	a4		4th	parameter	-
50	a5		5th	parameter	-
51	========	=================	================
52
53The return codes may be one of the following:
54
55	====		=========================
56	Code		Meaning
57	====		=========================
58	0		Success
59	-1		Hypercall not implemented
60	-2		Bad Hypercall parameter
61	====		=========================
62
63KVM Hypercalls Documentation
64============================
65
66The template for each hypercall is as follows:
67
681. Hypercall name
692. Purpose
70
711. KVM_HCALL_FUNC_IPI
72------------------------
73
74:Purpose: Send IPIs to multiple vCPUs.
75
76- a0: KVM_HCALL_FUNC_IPI
77- a1: Lower part of the bitmap for destination physical CPUIDs
78- a2: Higher part of the bitmap for destination physical CPUIDs
79- a3: The lowest physical CPUID in the bitmap
80
81The hypercall lets a guest send multiple IPIs (Inter-Process Interrupts) with
82at most 128 destinations per hypercall. The destinations are represented in a
83bitmap contained in the first two input registers (a1 and a2).
84
85Bit 0 of a1 corresponds to the physical CPUID in the third input register (a3)
86and bit 1 corresponds to the physical CPUID in a3+1, and so on.
87
88PV IPI on LoongArch includes both PV IPI multicast sending and PV IPI receiving,
89and SWI is used for PV IPI inject since there is no VM-exits accessing SWI registers.
90