1.. SPDX-License-Identifier: GPL-2.0 2 3========================= 4Introduction to LoongArch 5========================= 6 7LoongArch is a new RISC ISA, which is a bit like MIPS or RISC-V. There are 8currently 3 variants: a reduced 32-bit version (LA32R), a standard 32-bit 9version (LA32S) and a 64-bit version (LA64). There are 4 privilege levels 10(PLVs) defined in LoongArch: PLV0~PLV3, from high to low. Kernel runs at PLV0 11while applications run at PLV3. This document introduces the registers, basic 12instruction set, virtual memory and some other topics of LoongArch. 13 14Registers 15========= 16 17LoongArch registers include general purpose registers (GPRs), floating point 18registers (FPRs), vector registers (VRs) and control status registers (CSRs) 19used in privileged mode (PLV0). 20 21GPRs 22---- 23 24LoongArch has 32 GPRs ( ``$r0`` ~ ``$r31`` ); each one is 32-bit wide in LA32 25and 64-bit wide in LA64. ``$r0`` is hard-wired to zero, and the other registers 26are not architecturally special. (Except ``$r1``, which is hard-wired as the 27link register of the BL instruction.) 28 29The kernel uses a variant of the LoongArch register convention, as described in 30the LoongArch ELF psABI spec, in :ref:`References <loongarch-references>`: 31 32================= =============== =================== ============ 33Name Alias Usage Preserved 34 across calls 35================= =============== =================== ============ 36``$r0`` ``$zero`` Constant zero Unused 37``$r1`` ``$ra`` Return address No 38``$r2`` ``$tp`` TLS/Thread pointer Unused 39``$r3`` ``$sp`` Stack pointer Yes 40``$r4``-``$r11`` ``$a0``-``$a7`` Argument registers No 41``$r4``-``$r5`` ``$v0``-``$v1`` Return value No 42``$r12``-``$r20`` ``$t0``-``$t8`` Temp registers No 43``$r21`` ``$u0`` Percpu base address Unused 44``$r22`` ``$fp`` Frame pointer Yes 45``$r23``-``$r31`` ``$s0``-``$s8`` Static registers Yes 46================= =============== =================== ============ 47 48.. Note:: 49 The register ``$r21`` is reserved in the ELF psABI, but used by the Linux 50 kernel for storing the percpu base address. It normally has no ABI name, 51 but is called ``$u0`` in the kernel. You may also see ``$v0`` or ``$v1`` 52 in some old code,however they are deprecated aliases of ``$a0`` and ``$a1`` 53 respectively. 54 55FPRs 56---- 57 58LoongArch has 32 FPRs ( ``$f0`` ~ ``$f31`` ) when FPU is present. Each one is 5964-bit wide on the LA64 cores. 60 61The floating-point register convention is the same as described in the 62LoongArch ELF psABI spec: 63 64================= ================== =================== ============ 65Name Alias Usage Preserved 66 across calls 67================= ================== =================== ============ 68``$f0``-``$f7`` ``$fa0``-``$fa7`` Argument registers No 69``$f0``-``$f1`` ``$fv0``-``$fv1`` Return value No 70``$f8``-``$f23`` ``$ft0``-``$ft15`` Temp registers No 71``$f24``-``$f31`` ``$fs0``-``$fs7`` Static registers Yes 72================= ================== =================== ============ 73 74.. Note:: 75 You may see ``$fv0`` or ``$fv1`` in some old code, however they are 76 deprecated aliases of ``$fa0`` and ``$fa1`` respectively. 77 78VRs 79---- 80 81There are currently 2 vector extensions to LoongArch: 82 83- LSX (Loongson SIMD eXtension) with 128-bit vectors, 84- LASX (Loongson Advanced SIMD eXtension) with 256-bit vectors. 85 86LSX brings ``$v0`` ~ ``$v31`` while LASX brings ``$x0`` ~ ``$x31`` as the vector 87registers. 88 89The VRs overlap with FPRs: for example, on a core implementing LSX and LASX, 90the lower 128 bits of ``$x0`` is shared with ``$v0``, and the lower 64 bits of 91``$v0`` is shared with ``$f0``; same with all other VRs. 92 93CSRs 94---- 95 96CSRs can only be accessed from privileged mode (PLV0): 97 98================= ===================================== ============== 99Address Full Name Abbrev Name 100================= ===================================== ============== 1010x0 Current Mode Information CRMD 1020x1 Pre-exception Mode Information PRMD 1030x2 Extension Unit Enable EUEN 1040x3 Miscellaneous Control MISC 1050x4 Exception Configuration ECFG 1060x5 Exception Status ESTAT 1070x6 Exception Return Address ERA 1080x7 Bad (Faulting) Virtual Address BADV 1090x8 Bad (Faulting) Instruction Word BADI 1100xC Exception Entrypoint Address EENTRY 1110x10 TLB Index TLBIDX 1120x11 TLB Entry High-order Bits TLBEHI 1130x12 TLB Entry Low-order Bits 0 TLBELO0 1140x13 TLB Entry Low-order Bits 1 TLBELO1 1150x18 Address Space Identifier ASID 1160x19 Page Global Directory Address for PGDL 117 Lower-half Address Space 1180x1A Page Global Directory Address for PGDH 119 Higher-half Address Space 1200x1B Page Global Directory Address PGD 1210x1C Page Walk Control for Lower- PWCL 122 half Address Space 1230x1D Page Walk Control for Higher- PWCH 124 half Address Space 1250x1E STLB Page Size STLBPS 1260x1F Reduced Virtual Address Configuration RVACFG 1270x20 CPU Identifier CPUID 1280x21 Privileged Resource Configuration 1 PRCFG1 1290x22 Privileged Resource Configuration 2 PRCFG2 1300x23 Privileged Resource Configuration 3 PRCFG3 1310x30+n (0≤n≤15) Saved Data register SAVEn 1320x40 Timer Identifier TID 1330x41 Timer Configuration TCFG 1340x42 Timer Value TVAL 1350x43 Compensation of Timer Count CNTC 1360x44 Timer Interrupt Clearing TICLR 1370x60 LLBit Control LLBCTL 1380x80 Implementation-specific Control 1 IMPCTL1 1390x81 Implementation-specific Control 2 IMPCTL2 1400x88 TLB Refill Exception Entrypoint TLBRENTRY 141 Address 1420x89 TLB Refill Exception BAD (Faulting) TLBRBADV 143 Virtual Address 1440x8A TLB Refill Exception Return Address TLBRERA 1450x8B TLB Refill Exception Saved Data TLBRSAVE 146 Register 1470x8C TLB Refill Exception Entry Low-order TLBRELO0 148 Bits 0 1490x8D TLB Refill Exception Entry Low-order TLBRELO1 150 Bits 1 1510x8E TLB Refill Exception Entry High-order TLBEHI 152 Bits 1530x8F TLB Refill Exception Pre-exception TLBRPRMD 154 Mode Information 1550x90 Machine Error Control MERRCTL 1560x91 Machine Error Information 1 MERRINFO1 1570x92 Machine Error Information 2 MERRINFO2 1580x93 Machine Error Exception Entrypoint MERRENTRY 159 Address 1600x94 Machine Error Exception Return MERRERA 161 Address 1620x95 Machine Error Exception Saved Data MERRSAVE 163 Register 1640x98 Cache TAGs CTAG 1650x180+n (0≤n≤3) Direct Mapping Configuration Window n DMWn 1660x200+2n (0≤n≤31) Performance Monitor Configuration n PMCFGn 1670x201+2n (0≤n≤31) Performance Monitor Overall Counter n PMCNTn 1680x300 Memory Load/Store WatchPoint MWPC 169 Overall Control 1700x301 Memory Load/Store WatchPoint MWPS 171 Overall Status 1720x310+8n (0≤n≤7) Memory Load/Store WatchPoint n MWPnCFG1 173 Configuration 1 1740x311+8n (0≤n≤7) Memory Load/Store WatchPoint n MWPnCFG2 175 Configuration 2 1760x312+8n (0≤n≤7) Memory Load/Store WatchPoint n MWPnCFG3 177 Configuration 3 1780x313+8n (0≤n≤7) Memory Load/Store WatchPoint n MWPnCFG4 179 Configuration 4 1800x380 Instruction Fetch WatchPoint FWPC 181 Overall Control 1820x381 Instruction Fetch WatchPoint FWPS 183 Overall Status 1840x390+8n (0≤n≤7) Instruction Fetch WatchPoint n FWPnCFG1 185 Configuration 1 1860x391+8n (0≤n≤7) Instruction Fetch WatchPoint n FWPnCFG2 187 Configuration 2 1880x392+8n (0≤n≤7) Instruction Fetch WatchPoint n FWPnCFG3 189 Configuration 3 1900x393+8n (0≤n≤7) Instruction Fetch WatchPoint n FWPnCFG4 191 Configuration 4 1920x500 Debug Register DBG 1930x501 Debug Exception Return Address DERA 1940x502 Debug Exception Saved Data Register DSAVE 195================= ===================================== ============== 196 197ERA, TLBRERA, MERRERA and DERA are sometimes also known as EPC, TLBREPC, MERREPC 198and DEPC respectively. 199 200Basic Instruction Set 201===================== 202 203Instruction formats 204------------------- 205 206LoongArch instructions are 32 bits wide, belonging to 9 basic instruction 207formats (and variants of them): 208 209=========== ========================== 210Format name Composition 211=========== ========================== 2122R Opcode + Rj + Rd 2133R Opcode + Rk + Rj + Rd 2144R Opcode + Ra + Rk + Rj + Rd 2152RI8 Opcode + I8 + Rj + Rd 2162RI12 Opcode + I12 + Rj + Rd 2172RI14 Opcode + I14 + Rj + Rd 2182RI16 Opcode + I16 + Rj + Rd 2191RI21 Opcode + I21L + Rj + I21H 220I26 Opcode + I26L + I26H 221=========== ========================== 222 223Rd is the destination register operand, while Rj, Rk and Ra ("a" stands for 224"additional") are the source register operands. I8/I12/I14/I16/I21/I26 are 225immediate operands of respective width. The longer I21 and I26 are stored 226in separate higher and lower parts in the instruction word, denoted by the "L" 227and "H" suffixes. 228 229List of Instructions 230-------------------- 231 232For brevity, only instruction names (mnemonics) are listed here; please see the 233:ref:`References <loongarch-references>` for details. 234 235 2361. Arithmetic Instructions:: 237 238 ADD.W SUB.W ADDI.W ADD.D SUB.D ADDI.D 239 SLT SLTU SLTI SLTUI 240 AND OR NOR XOR ANDN ORN ANDI ORI XORI 241 MUL.W MULH.W MULH.WU DIV.W DIV.WU MOD.W MOD.WU 242 MUL.D MULH.D MULH.DU DIV.D DIV.DU MOD.D MOD.DU 243 PCADDI PCADDU12I PCADDU18I 244 LU12I.W LU32I.D LU52I.D ADDU16I.D 245 2462. Bit-shift Instructions:: 247 248 SLL.W SRL.W SRA.W ROTR.W SLLI.W SRLI.W SRAI.W ROTRI.W 249 SLL.D SRL.D SRA.D ROTR.D SLLI.D SRLI.D SRAI.D ROTRI.D 250 2513. Bit-manipulation Instructions:: 252 253 EXT.W.B EXT.W.H CLO.W CLO.D SLZ.W CLZ.D CTO.W CTO.D CTZ.W CTZ.D 254 BYTEPICK.W BYTEPICK.D BSTRINS.W BSTRINS.D BSTRPICK.W BSTRPICK.D 255 REVB.2H REVB.4H REVB.2W REVB.D REVH.2W REVH.D BITREV.4B BITREV.8B BITREV.W BITREV.D 256 MASKEQZ MASKNEZ 257 2584. Branch Instructions:: 259 260 BEQ BNE BLT BGE BLTU BGEU BEQZ BNEZ B BL JIRL 261 2625. Load/Store Instructions:: 263 264 LD.B LD.BU LD.H LD.HU LD.W LD.WU LD.D ST.B ST.H ST.W ST.D 265 LDX.B LDX.BU LDX.H LDX.HU LDX.W LDX.WU LDX.D STX.B STX.H STX.W STX.D 266 LDPTR.W LDPTR.D STPTR.W STPTR.D 267 PRELD PRELDX 268 2696. Atomic Operation Instructions:: 270 271 LL.W SC.W LL.D SC.D 272 AMSWAP.W AMSWAP.D AMADD.W AMADD.D AMAND.W AMAND.D AMOR.W AMOR.D AMXOR.W AMXOR.D 273 AMMAX.W AMMAX.D AMMIN.W AMMIN.D 274 2757. Barrier Instructions:: 276 277 IBAR DBAR 278 2798. Special Instructions:: 280 281 SYSCALL BREAK CPUCFG NOP IDLE ERTN(ERET) DBCL(DBGCALL) RDTIMEL.W RDTIMEH.W RDTIME.D 282 ASRTLE.D ASRTGT.D 283 2849. Privileged Instructions:: 285 286 CSRRD CSRWR CSRXCHG 287 IOCSRRD.B IOCSRRD.H IOCSRRD.W IOCSRRD.D IOCSRWR.B IOCSRWR.H IOCSRWR.W IOCSRWR.D 288 CACOP TLBP(TLBSRCH) TLBRD TLBWR TLBFILL TLBCLR TLBFLUSH INVTLB LDDIR LDPTE 289 290Virtual Memory 291============== 292 293LoongArch supports direct-mapped virtual memory and page-mapped virtual memory. 294 295Direct-mapped virtual memory is configured by CSR.DMWn (n=0~3), it has a simple 296relationship between virtual address (VA) and physical address (PA):: 297 298 VA = PA + FixedOffset 299 300Page-mapped virtual memory has arbitrary relationship between VA and PA, which 301is recorded in TLB and page tables. LoongArch's TLB includes a fully-associative 302MTLB (Multiple Page Size TLB) and set-associative STLB (Single Page Size TLB). 303 304By default, the whole virtual address space of LA32 is configured like this: 305 306============ =========================== ============================= 307Name Address Range Attributes 308============ =========================== ============================= 309``UVRANGE`` ``0x00000000 - 0x7FFFFFFF`` Page-mapped, Cached, PLV0~3 310``KPRANGE0`` ``0x80000000 - 0x9FFFFFFF`` Direct-mapped, Uncached, PLV0 311``KPRANGE1`` ``0xA0000000 - 0xBFFFFFFF`` Direct-mapped, Cached, PLV0 312``KVRANGE`` ``0xC0000000 - 0xFFFFFFFF`` Page-mapped, Cached, PLV0 313============ =========================== ============================= 314 315User mode (PLV3) can only access UVRANGE. For direct-mapped KPRANGE0 and 316KPRANGE1, PA is equal to VA with bit30~31 cleared. For example, the uncached 317direct-mapped VA of 0x00001000 is 0x80001000, and the cached direct-mapped 318VA of 0x00001000 is 0xA0001000. 319 320By default, the whole virtual address space of LA64 is configured like this: 321 322============ ====================== ====================================== 323Name Address Range Attributes 324============ ====================== ====================================== 325``XUVRANGE`` ``0x0000000000000000 - Page-mapped, Cached, PLV0~3 326 0x3FFFFFFFFFFFFFFF`` 327``XSPRANGE`` ``0x4000000000000000 - Direct-mapped, Cached / Uncached, PLV0 328 0x7FFFFFFFFFFFFFFF`` 329``XKPRANGE`` ``0x8000000000000000 - Direct-mapped, Cached / Uncached, PLV0 330 0xBFFFFFFFFFFFFFFF`` 331``XKVRANGE`` ``0xC000000000000000 - Page-mapped, Cached, PLV0 332 0xFFFFFFFFFFFFFFFF`` 333============ ====================== ====================================== 334 335User mode (PLV3) can only access XUVRANGE. For direct-mapped XSPRANGE and 336XKPRANGE, PA is equal to VA with bits 60~63 cleared, and the cache attribute 337is configured by bits 60~61 in VA: 0 is for strongly-ordered uncached, 1 is 338for coherent cached, and 2 is for weakly-ordered uncached. 339 340Currently we only use XKPRANGE for direct mapping and XSPRANGE is reserved. 341 342To put this in action: the strongly-ordered uncached direct-mapped VA (in 343XKPRANGE) of 0x00000000_00001000 is 0x80000000_00001000, the coherent cached 344direct-mapped VA (in XKPRANGE) of 0x00000000_00001000 is 0x90000000_00001000, 345and the weakly-ordered uncached direct-mapped VA (in XKPRANGE) of 0x00000000 346_00001000 is 0xA0000000_00001000. 347 348Relationship of Loongson and LoongArch 349====================================== 350 351LoongArch is a RISC ISA which is different from any other existing ones, while 352Loongson is a family of processors. Loongson includes 3 series: Loongson-1 is 353the 32-bit processor series, Loongson-2 is the low-end 64-bit processor series, 354and Loongson-3 is the high-end 64-bit processor series. Old Loongson is based on 355MIPS, while New Loongson is based on LoongArch. Take Loongson-3 as an example: 356Loongson-3A1000/3B1500/3A2000/3A3000/3A4000 are MIPS-compatible, while Loongson- 3573A5000 (and future revisions) are all based on LoongArch. 358 359.. _loongarch-references: 360 361References 362========== 363 364Official web site of Loongson Technology Corp. Ltd.: 365 366 http://www.loongson.cn/ 367 368Developer web site of Loongson and LoongArch (Software and Documentation): 369 370 http://www.loongnix.cn/ 371 372 https://github.com/loongson/ 373 374 https://loongson.github.io/LoongArch-Documentation/ 375 376Documentation of LoongArch ISA: 377 378 https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-Vol1-v1.10-CN.pdf (in Chinese) 379 380 https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-Vol1-v1.10-EN.pdf (in English) 381 382Documentation of LoongArch ELF psABI: 383 384 https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-ELF-ABI-v2.01-CN.pdf (in Chinese) 385 386 https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-ELF-ABI-v2.01-EN.pdf (in English) 387 388Linux kernel repository of Loongson and LoongArch: 389 390 https://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson.git 391