Documentation: KVM: Add hypercall for LoongArch

Upstream: no

Add documentation topic for using pv_virt when running as a guest
on KVM hypervisor.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Xianglai Li <lixianglai@loongson.cn>

Documentation/virt/kvm/index.rst

@@ -14,6 +14,7 @@ KVM
    s390/index
    ppc-pv
    x86/index
+   loongarch/index
 
    locking
    vcpu-requests

Documentation/virt/kvm/loongarch/hypercalls.rst

@@ -0,0 +1,79 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+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 "KVM\0"
+
+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.

Documentation/virt/kvm/loongarch/index.rst

@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+KVM for LoongArch systems
+=========================
+
+.. toctree::
+   :maxdepth: 2
+
+   hypercalls.rst