ARK-OS/arkos/kernel/Documentation/arch/loongarch/introduction.rst

.. SPDX-License-Identifier: GPL-2.0

=========================
Introduction to LoongArch
=========================

LoongArch is a new RISC ISA, which is a bit like MIPS or RISC-V. There are
currently 3 variants: a reduced 32-bit version (LA32R), a standard 32-bit
version (LA32S) and a 64-bit version (LA64). There are 4 privilege levels
(PLVs) defined in LoongArch: PLV0~PLV3, from high to low. Kernel runs at PLV0
while applications run at PLV3. This document introduces the registers, basic
instruction set, virtual memory and some other topics of LoongArch.

Registers
=========

LoongArch registers include general purpose registers (GPRs), floating point
registers (FPRs), vector registers (VRs) and control status registers (CSRs)
used in privileged mode (PLV0).

GPRs
----

LoongArch has 32 GPRs ( ``$r0`` ~ ``$r31`` ); each one is 32-bit wide in LA32
and 64-bit wide in LA64. ``$r0`` is hard-wired to zero, and the other registers
are not architecturally special. (Except ``$r1``, which is hard-wired as the
link register of the BL instruction.)

The kernel uses a variant of the LoongArch register convention, as described in
the LoongArch ELF psABI spec, in :ref:`References <loongarch-references>`:

================= =============== =================== ============
Name              Alias           Usage               Preserved
                                                      across calls
================= =============== =================== ============
``$r0``           ``$zero``       Constant zero       Unused
``$r1``           ``$ra``         Return address      No
``$r2``           ``$tp``         TLS/Thread pointer  Unused
``$r3``           ``$sp``         Stack pointer       Yes
``$r4``-``$r11``  ``$a0``-``$a7`` Argument registers  No
``$r4``-``$r5``   ``$v0``-``$v1`` Return value        No
``$r12``-``$r20`` ``$t0``-``$t8`` Temp registers      No
``$r21``          ``$u0``         Percpu base address Unused
``$r22``          ``$fp``         Frame pointer       Yes
``$r23``-``$r31`` ``$s0``-``$s8`` Static registers    Yes
================= =============== =================== ============

.. Note::
    The register ``$r21`` is reserved in the ELF psABI, but used by the Linux
    kernel for storing the percpu base address. It normally has no ABI name,
    but is called ``$u0`` in the kernel. You may also see ``$v0`` or ``$v1``
    in some old code,however they are deprecated aliases of ``$a0`` and ``$a1``
    respectively.

FPRs
----

LoongArch has 32 FPRs ( ``$f0`` ~ ``$f31`` ) when FPU is present. Each one is
64-bit wide on the LA64 cores.

The floating-point register convention is the same as described in the
LoongArch ELF psABI spec:

================= ================== =================== ============
Name              Alias              Usage               Preserved
                                                         across calls
================= ================== =================== ============
``$f0``-``$f7``   ``$fa0``-``$fa7``  Argument registers  No
``$f0``-``$f1``   ``$fv0``-``$fv1``  Return value        No
``$f8``-``$f23``  ``$ft0``-``$ft15`` Temp registers      No
``$f24``-``$f31`` ``$fs0``-``$fs7``  Static registers    Yes
================= ================== =================== ============

.. Note::
    You may see ``$fv0`` or ``$fv1`` in some old code, however they are
    deprecated aliases of ``$fa0`` and ``$fa1`` respectively.

VRs
----

There are currently 2 vector extensions to LoongArch:

- LSX (Loongson SIMD eXtension) with 128-bit vectors,
- LASX (Loongson Advanced SIMD eXtension) with 256-bit vectors.

LSX brings ``$v0`` ~ ``$v31`` while LASX brings ``$x0`` ~ ``$x31`` as the vector
registers.

The VRs overlap with FPRs: for example, on a core implementing LSX and LASX,
the lower 128 bits of ``$x0`` is shared with ``$v0``, and the lower 64 bits of
``$v0`` is shared with ``$f0``; same with all other VRs.

CSRs
----

CSRs can only be accessed from privileged mode (PLV0):

================= ===================================== ==============
Address           Full Name                             Abbrev Name
================= ===================================== ==============
0x0               Current Mode Information              CRMD
0x1               Pre-exception Mode Information        PRMD
0x2               Extension Unit Enable                 EUEN
0x3               Miscellaneous Control                 MISC
0x4               Exception Configuration               ECFG
0x5               Exception Status                      ESTAT
0x6               Exception Return Address              ERA
0x7               Bad (Faulting) Virtual Address        BADV
0x8               Bad (Faulting) Instruction Word       BADI
0xC               Exception Entrypoint Address          EENTRY
0x10              TLB Index                             TLBIDX
0x11              TLB Entry High-order Bits             TLBEHI
0x12              TLB Entry Low-order Bits 0            TLBELO0
0x13              TLB Entry Low-order Bits 1            TLBELO1
0x18              Address Space Identifier              ASID
0x19              Page Global Directory Address for     PGDL
                  Lower-half Address Space
0x1A              Page Global Directory Address for     PGDH
                  Higher-half Address Space
0x1B              Page Global Directory Address         PGD
0x1C              Page Walk Control for Lower-          PWCL
                  half Address Space
0x1D              Page Walk Control for Higher-         PWCH
                  half Address Space
0x1E              STLB Page Size                        STLBPS
0x1F              Reduced Virtual Address Configuration RVACFG
0x20              CPU Identifier                        CPUID
0x21              Privileged Resource Configuration 1   PRCFG1
0x22              Privileged Resource Configuration 2   PRCFG2
0x23              Privileged Resource Configuration 3   PRCFG3
0x30+n (0≤n≤15)   Saved Data register                   SAVEn
0x40              Timer Identifier                      TID
0x41              Timer Configuration                   TCFG
0x42              Timer Value                           TVAL
0x43              Compensation of Timer Count           CNTC
0x44              Timer Interrupt Clearing              TICLR
0x60              LLBit Control                         LLBCTL
0x80              Implementation-specific Control 1     IMPCTL1
0x81              Implementation-specific Control 2     IMPCTL2
0x88              TLB Refill Exception Entrypoint       TLBRENTRY
                  Address
0x89              TLB Refill Exception BAD (Faulting)   TLBRBADV
                  Virtual Address
0x8A              TLB Refill Exception Return Address   TLBRERA
0x8B              TLB Refill Exception Saved Data       TLBRSAVE
                  Register
0x8C              TLB Refill Exception Entry Low-order  TLBRELO0
                  Bits 0
0x8D              TLB Refill Exception Entry Low-order  TLBRELO1
                  Bits 1
0x8E              TLB Refill Exception Entry High-order TLBEHI
                  Bits
0x8F              TLB Refill Exception Pre-exception    TLBRPRMD
                  Mode Information
0x90              Machine Error Control                 MERRCTL
0x91              Machine Error Information 1           MERRINFO1
0x92              Machine Error Information 2           MERRINFO2
0x93              Machine Error Exception Entrypoint    MERRENTRY
                  Address
0x94              Machine Error Exception Return        MERRERA
                  Address
0x95              Machine Error Exception Saved Data    MERRSAVE
                  Register
0x98              Cache TAGs                            CTAG
0x180+n (0≤n≤3)   Direct Mapping Configuration Window n DMWn
0x200+2n (0≤n≤31) Performance Monitor Configuration n   PMCFGn
0x201+2n (0≤n≤31) Performance Monitor Overall Counter n PMCNTn
0x300             Memory Load/Store WatchPoint          MWPC
                  Overall Control
0x301             Memory Load/Store WatchPoint          MWPS
                  Overall Status
0x310+8n (0≤n≤7)  Memory Load/Store WatchPoint n        MWPnCFG1
                  Configuration 1
0x311+8n (0≤n≤7)  Memory Load/Store WatchPoint n        MWPnCFG2
                  Configuration 2
0x312+8n (0≤n≤7)  Memory Load/Store WatchPoint n        MWPnCFG3
                  Configuration 3
0x313+8n (0≤n≤7)  Memory Load/Store WatchPoint n        MWPnCFG4
                  Configuration 4
0x380             Instruction Fetch WatchPoint          FWPC
                  Overall Control
0x381             Instruction Fetch WatchPoint          FWPS
                  Overall Status
0x390+8n (0≤n≤7)  Instruction Fetch WatchPoint n        FWPnCFG1
                  Configuration 1
0x391+8n (0≤n≤7)  Instruction Fetch WatchPoint n        FWPnCFG2
                  Configuration 2
0x392+8n (0≤n≤7)  Instruction Fetch WatchPoint n        FWPnCFG3
                  Configuration 3
0x393+8n (0≤n≤7)  Instruction Fetch WatchPoint n        FWPnCFG4
                  Configuration 4
0x500             Debug Register                        DBG
0x501             Debug Exception Return Address        DERA
0x502             Debug Exception Saved Data Register   DSAVE
================= ===================================== ==============

ERA, TLBRERA, MERRERA and DERA are sometimes also known as EPC, TLBREPC, MERREPC
and DEPC respectively.

Basic Instruction Set
=====================

Instruction formats
-------------------

LoongArch instructions are 32 bits wide, belonging to 9 basic instruction
formats (and variants of them):

=========== ==========================
Format name Composition
=========== ==========================
2R          Opcode + Rj + Rd
3R          Opcode + Rk + Rj + Rd
4R          Opcode + Ra + Rk + Rj + Rd
2RI8        Opcode + I8 + Rj + Rd
2RI12       Opcode + I12 + Rj + Rd
2RI14       Opcode + I14 + Rj + Rd
2RI16       Opcode + I16 + Rj + Rd
1RI21       Opcode + I21L + Rj + I21H
I26         Opcode + I26L + I26H
=========== ==========================

Rd is the destination register operand, while Rj, Rk and Ra ("a" stands for
"additional") are the source register operands. I8/I12/I14/I16/I21/I26 are
immediate operands of respective width. The longer I21 and I26 are stored
in separate higher and lower parts in the instruction word, denoted by the "L"
and "H" suffixes.

List of Instructions
--------------------

For brevity, only instruction names (mnemonics) are listed here; please see the
:ref:`References <loongarch-references>` for details.


1. Arithmetic Instructions::

    ADD.W SUB.W ADDI.W ADD.D SUB.D ADDI.D
    SLT SLTU SLTI SLTUI
    AND OR NOR XOR ANDN ORN ANDI ORI XORI
    MUL.W MULH.W MULH.WU DIV.W DIV.WU MOD.W MOD.WU
    MUL.D MULH.D MULH.DU DIV.D DIV.DU MOD.D MOD.DU
    PCADDI PCADDU12I PCADDU18I
    LU12I.W LU32I.D LU52I.D ADDU16I.D

2. Bit-shift Instructions::

    SLL.W SRL.W SRA.W ROTR.W SLLI.W SRLI.W SRAI.W ROTRI.W
    SLL.D SRL.D SRA.D ROTR.D SLLI.D SRLI.D SRAI.D ROTRI.D

3. Bit-manipulation Instructions::

    EXT.W.B EXT.W.H CLO.W CLO.D SLZ.W CLZ.D CTO.W CTO.D CTZ.W CTZ.D
    BYTEPICK.W BYTEPICK.D BSTRINS.W BSTRINS.D BSTRPICK.W BSTRPICK.D
    REVB.2H REVB.4H REVB.2W REVB.D REVH.2W REVH.D BITREV.4B BITREV.8B BITREV.W BITREV.D
    MASKEQZ MASKNEZ

4. Branch Instructions::

    BEQ BNE BLT BGE BLTU BGEU BEQZ BNEZ B BL JIRL

5. Load/Store Instructions::

    LD.B LD.BU LD.H LD.HU LD.W LD.WU LD.D ST.B ST.H ST.W ST.D
    LDX.B LDX.BU LDX.H LDX.HU LDX.W LDX.WU LDX.D STX.B STX.H STX.W STX.D
    LDPTR.W LDPTR.D STPTR.W STPTR.D
    PRELD PRELDX

6. Atomic Operation Instructions::

    LL.W SC.W LL.D SC.D
    AMSWAP.W AMSWAP.D AMADD.W AMADD.D AMAND.W AMAND.D AMOR.W AMOR.D AMXOR.W AMXOR.D
    AMMAX.W AMMAX.D AMMIN.W AMMIN.D

7. Barrier Instructions::

    IBAR DBAR

8. Special Instructions::

    SYSCALL BREAK CPUCFG NOP IDLE ERTN(ERET) DBCL(DBGCALL) RDTIMEL.W RDTIMEH.W RDTIME.D
    ASRTLE.D ASRTGT.D

9. Privileged Instructions::

    CSRRD CSRWR CSRXCHG
    IOCSRRD.B IOCSRRD.H IOCSRRD.W IOCSRRD.D IOCSRWR.B IOCSRWR.H IOCSRWR.W IOCSRWR.D
    CACOP TLBP(TLBSRCH) TLBRD TLBWR TLBFILL TLBCLR TLBFLUSH INVTLB LDDIR LDPTE

Virtual Memory
==============

LoongArch supports direct-mapped virtual memory and page-mapped virtual memory.

Direct-mapped virtual memory is configured by CSR.DMWn (n=0~3), it has a simple
relationship between virtual address (VA) and physical address (PA)::

 VA = PA + FixedOffset

Page-mapped virtual memory has arbitrary relationship between VA and PA, which
is recorded in TLB and page tables. LoongArch's TLB includes a fully-associative
MTLB (Multiple Page Size TLB) and set-associative STLB (Single Page Size TLB).

By default, the whole virtual address space of LA32 is configured like this:

============ =========================== =============================
Name         Address Range               Attributes
============ =========================== =============================
``UVRANGE``  ``0x00000000 - 0x7FFFFFFF`` Page-mapped, Cached, PLV0~3
``KPRANGE0`` ``0x80000000 - 0x9FFFFFFF`` Direct-mapped, Uncached, PLV0
``KPRANGE1`` ``0xA0000000 - 0xBFFFFFFF`` Direct-mapped, Cached, PLV0
``KVRANGE``  ``0xC0000000 - 0xFFFFFFFF`` Page-mapped, Cached, PLV0
============ =========================== =============================

User mode (PLV3) can only access UVRANGE. For direct-mapped KPRANGE0 and
KPRANGE1, PA is equal to VA with bit30~31 cleared. For example, the uncached
direct-mapped VA of 0x00001000 is 0x80001000, and the cached direct-mapped
VA of 0x00001000 is 0xA0001000.

By default, the whole virtual address space of LA64 is configured like this:

============ ====================== ======================================
Name         Address Range          Attributes
============ ====================== ======================================
``XUVRANGE`` ``0x0000000000000000 - Page-mapped, Cached, PLV0~3
             0x3FFFFFFFFFFFFFFF``
``XSPRANGE`` ``0x4000000000000000 - Direct-mapped, Cached / Uncached, PLV0
             0x7FFFFFFFFFFFFFFF``
``XKPRANGE`` ``0x8000000000000000 - Direct-mapped, Cached / Uncached, PLV0
             0xBFFFFFFFFFFFFFFF``
``XKVRANGE`` ``0xC000000000000000 - Page-mapped, Cached, PLV0
             0xFFFFFFFFFFFFFFFF``
============ ====================== ======================================

User mode (PLV3) can only access XUVRANGE. For direct-mapped XSPRANGE and
XKPRANGE, PA is equal to VA with bits 60~63 cleared, and the cache attribute
is configured by bits 60~61 in VA: 0 is for strongly-ordered uncached, 1 is
for coherent cached, and 2 is for weakly-ordered uncached.

Currently we only use XKPRANGE for direct mapping and XSPRANGE is reserved.

To put this in action: the strongly-ordered uncached direct-mapped VA (in
XKPRANGE) of 0x00000000_00001000 is 0x80000000_00001000, the coherent cached
direct-mapped VA (in XKPRANGE) of 0x00000000_00001000 is 0x90000000_00001000,
and the weakly-ordered uncached direct-mapped VA (in XKPRANGE) of 0x00000000
_00001000 is 0xA0000000_00001000.

Relationship of Loongson and LoongArch
======================================

LoongArch is a RISC ISA which is different from any other existing ones, while
Loongson is a family of processors. Loongson includes 3 series: Loongson-1 is
the 32-bit processor series, Loongson-2 is the low-end 64-bit processor series,
and Loongson-3 is the high-end 64-bit processor series. Old Loongson is based on
MIPS, while New Loongson is based on LoongArch. Take Loongson-3 as an example:
Loongson-3A1000/3B1500/3A2000/3A3000/3A4000 are MIPS-compatible, while Loongson-
3A5000 (and future revisions) are all based on LoongArch.

.. _loongarch-references:

References
==========

Official web site of Loongson Technology Corp. Ltd.:

  http://www.loongson.cn/

Developer web site of Loongson and LoongArch (Software and Documentation):

  http://www.loongnix.cn/

  https://github.com/loongson/

  https://loongson.github.io/LoongArch-Documentation/

Documentation of LoongArch ISA:

  https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-Vol1-v1.10-CN.pdf (in Chinese)

  https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-Vol1-v1.10-EN.pdf (in English)

Documentation of LoongArch ELF psABI:

  https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-ELF-ABI-v2.01-CN.pdf (in Chinese)

  https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-ELF-ABI-v2.01-EN.pdf (in English)

Linux kernel repository of Loongson and LoongArch:

  https://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson.git