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