Loongarch: KVM: Add KVM hypercalls documentation for LoongArch

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>
Co-developed-by: Mingcong Bai <jeffbai@aosc.io>
Signed-off-by: Mingcong Bai <jeffbai@aosc.io>
Link: https://lore.kernel.org/all/5c338084b1bcccc1d57dce9ddb1e7081@aosc.io/Signed-off-by: Dandan Zhang <zhangdandan@uniontech.com>
[jc: fixed htmldocs build error]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/4769C036576F8816+20240828045950.3484113-1-zhangdandan@uniontech.com

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

+.. 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. Up to five arguments may be placed in registers a1 - a5.
+The return value is placed in v0 (an alias of a0).
+
+Source code for this interface can be found in arch/loongarch/kvm*.
+
+Querying for existence
+======================
+
+To determine if the host is running on KVM, we can utilize the cpucfg()
+function at index CPUCFG_KVM_BASE (0x40000000).
+
+The CPUCFG_KVM_BASE range, spanning from 0x40000000 to 0x400000FF, The
+CPUCFG_KVM_BASE range between 0x40000000 - 0x400000FF is marked as reserved.
+Consequently, all current and future processors will not implement any
+feature within this range.
+
+On a KVM-virtualized Linux system, a read operation on cpucfg() at index
+CPUCFG_KVM_BASE (0x40000000) returns the magic string 'KVM\0'.
+
+Once you have determined that your host is running on a paravirtualization-
+capable KVM, you may now use hypercalls as described below.
+
+KVM hypercall ABI
+=================
+
+The KVM hypercall ABI is simple, with one scratch register a0 (v0) and at most
+five generic registers (a1 - a5) used as input parameters. The FP (Floating-
+point) and vector registers are not utilized as input registers and must
+remain unmodified during a hypercall.
+
+Hypercall functions can be inlined as it only uses 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	-
+	========	=================	================
+
+The return codes may be one of the following:
+
+	====		=========================
+	Code		Meaning
+	====		=========================
+	0		Success
+	-1		Hypercall not implemented
+	-2		Bad Hypercall parameter
+	====		=========================
+
+KVM Hypercalls Documentation
+============================
+
+The template for each hypercall is as follows:
+
+1. Hypercall name
+2. Purpose
+
+1. KVM_HCALL_FUNC_IPI
+------------------------
+
+:Purpose: Send IPIs to multiple vCPUs.
+
+- a0: KVM_HCALL_FUNC_IPI
+- a1: Lower part of the bitmap for destination physical CPUIDs
+- a2: Higher part of the bitmap for destination physical CPUIDs
+- a3: The lowest physical CPUID in the bitmap
+
+The hypercall lets a guest send multiple IPIs (Inter-Process Interrupts) with
+at most 128 destinations per hypercall. The destinations are represented in a
+bitmap contained in the first two input registers (a1 and a2).
+
+Bit 0 of a1 corresponds to the physical CPUID in the third input register (a3)
+and bit 1 corresponds to the physical CPUID in a3+1, and so on.
+
+PV IPI on LoongArch includes both PV IPI multicast sending and PV IPI receiving,
+and SWI is used for PV IPI inject since there is no VM-exits accessing SWI registers.

Documentation/virt/kvm/loongarch/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+KVM for LoongArch systems
+=========================
+
+.. toctree::
+   :maxdepth: 2
+
+   hypercalls.rst

MAINTAINERS

@@ -12299,6 +12299,7 @@ L:	kvm@vger.kernel.org
 L:	loongarch@lists.linux.dev
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/virt/kvm/kvm.git
+F:	Documentation/virt/kvm/loongarch/
 F:	arch/loongarch/include/asm/kvm*
 F:	arch/loongarch/include/uapi/asm/kvm*
 F:	arch/loongarch/kvm/