The LoongArch paravirtual interface

KVM hypercalls use the HVCL instruction with code 0x100, and the hypercall number is put in a0 and up to five arguments may be placed in a1-a5, the return value is placed in v0 (alias with a0).

The code for that interface can be found in arch/loongarch/kvm/*

Querying for existence

To find out if we’re running on KVM or not, cpucfg can be used with index CPUCFG_KVM_BASE (0x40000000), cpucfg range between 0x40000000 - 0x400000FF is marked as a specially reserved range. All existing and future processors will not implement any features in this range.

When Linux is running on KVM, cpucfg with index CPUCFG_KVM_BASE (0x40000000) returns magic string “KVM0”

Once you determined you’re running under a PV capable KVM, you can now use hypercalls as described below.

KVM hypercall ABI

Hypercall ABI on KVM is simple, only one scratch register a0 (v0) and at most five generic registers used as input parameter. FP register and vector register is not used for input register and should not be modified during hypercall. Hypercall function can be inlined since there is only one scratch register.

The parameters are as follows:

Register

IN

OUT

a0

function number

Return code

a1

1st parameter

a2

2nd parameter

a3

3rd parameter

a4

4th parameter

a5

5th parameter

Return codes can be as follows:

Code

Meaning

0

Success

-1

Hypercall not implemented

-2

Hypercall parameter error

KVM Hypercalls Documentation

The template for each hypercall is: 1. Hypercall name 2. Purpose

1. KVM_HCALL_FUNC_PV_IPI

Purpose:

Send IPIs to multiple vCPUs.

  • a0: KVM_HCALL_FUNC_PV_IPI

  • a1: lower part of the bitmap of destination physical CPUIDs

  • a2: higher part of the bitmap of destination physical CPUIDs

  • a3: the lowest physical CPUID in bitmap

The hypercall lets a guest send multicast IPIs, with at most 128 destinations per hypercall. The destinations are represented by a bitmap contained in the first two arguments (a1 and a2). Bit 0 of a1 corresponds to the physical CPUID in the third argument (a3), bit 1 corresponds to the physical ID a3+1, and so on.