.. 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-v1.02-CN.pdf (in Chinese)
https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-Vol1-v1.02-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