overloadable_opcodes.mdwn

   1 # Overloadable opcodes.
   2
   3 This proposal adds a standardised extension instructions to the RV
   4 instruction set by introducing a fixed small number N (e.g. N = 8) of
   5 R-type opcodes xcmd0 rd, rs1, rs2, .. , xcmd[N-1] rd, rs1, rs2, that are intended to be used as "overloadable" R-type instructions for independently developed extensions. Extensions may be implemented in the form of non standard CPU extensions, IP tiles, or closely coupled external devices.
   6
   7 Tl;DR see below for a C description of how this is supposed to work.
   8
   9 -----
  10     xcmd0 rd, rs1, rs2
  11     xcmd1 rd, rs1, rs2
  12     ....
  13     xcmdN rd, rs1, rs2
  14
  15 * rs1 contains a 12 bit "logical unit" (lun)  together with xlen - 12 bits of additional data.
  16 * rs2 is arbitrary
  17
  18 For xmd3, route the inputs rs1, rs2 and output port rd to command 3 of the (sub)device on the cpu identified by the lun bits of rs1.
  19
  20 after execution
  21 * rd contains the value that of the output port of the implementing device
  22
  23 --------
  24     xext  rd, rs1, rs2
  25     xext0 rd, rs1, rs2
  26     xextm1 rd, rs1, rs2
  27
  28
  29 * rs1 contains
  30 --a UUID of at least 20 bit in bit 12 .. XLEN of rs1 identifying an xintf.
  31 --the sequence number of a device at the current privilege level on the cpu implementing the xintf in bit 0..11 .
  32   In particular, if bit 0..11  is zero, the default implemententation is meant.
  33 * rs2 is arbitrary (but bit XLEN-12 to XLEN -1 are unused)
  34
  35 after execution,
  36   rd contains the lun of a device implementing the xintf or the luns 0 (xext), 1 (xext0) or 2 (xextm1)
  37
  38 ---
  39 The net effect is that, when the CPU implements an xintf with UUID 0xABCDE a sequence like
  40
  41     //fake UUID of an xintf
  42     lui   rd 0xABCDE
  43     xext  rd rd rs1
  44     xcmd0 rd rd rs2
  45
  46 acts like a single namespaced instruction cmd0_ABCDE rd rs1 rs2 with the annoying caveat that rs1 can only use bits 0..XLEN-12 (the sequence is also not indivisible but the crucial semantics that you might want to be indivisible is in xcmd0). Delegation is expected to come at a small
  47 additional performance price compared to a "native" instruction. This should, however, be an acceptable tradeoff in many cases. Moreover implementations may opcode fuse the whole instruction sequence (or the first or last two instructions).
  48
  49 If several instructions of the same interface are used, one can also use instruction sequences like
  50
  51     lui   t1 0xABCDE  //org_tinker_tinker__RocknRoll_uuid
  52     xext  t1 t1 zero
  53     xcmd0 a5, t1, a0  // org_tinker_tinker__RocknRoll__rock(a5, t1, a0)
  54     xcmd1 t2, t1, a1  // org_tinker_tinker__RocknRoll__roll(t2, t1, a5)
  55     xcmd0 a0, t1, t2  // org_tinker_tinker__RocknRoll__rock(a0, t1, t2)
  56
  57 This amortises the cost of the xext instruction.
  58
  59 When the xintf UUID is not recognised, the xcmd in the above sequence traps. Using xext0 instead of xext ensures that the xcmd0 returns 0.
  60 Likewise, using xextm1 ensures that the xcmd returns -1. This requires lun = 0 , 1 and 2 to be routed to three mandatory fallback
  61 interfaces defined below.
  62
  63 On the software level, the xintf is just a set of glorified assembler macros
  64
  65      org.tinker.tinker:RocknRoll{
  66         uuid : 0xABCDE
  67         rock rd rs1 rs2 : xcmd0 rd rs1 rs2
  68         roll rd rs1 rs2 : xcmd1 rd rs1 rs2
  69      }
  70
  71 so that the above sequence can be more clearly written as
  72
  73     import(org.tinker.tinker:RocknRoll)
  74
  75     lui rd org.tinker.tinker:RocknRoll:uuid
  76     xext rd rd rs1
  77     org.tinker.tinker:RocknRoll:rock rd rd rs2
  78
  79
  80 ------
  81 The following standard xintfs shall be implemented by the CPU.
  82
  83 For lun == 0:
  84
  85 At  privilege level user mode, supervisor mode and hypervisor mode
  86
  87     org.RiscV:Fallback:Trap{
  88         uuid: 0
  89         trap0 rd rs1 rs2: xcmd0 rd rs1 rs2
  90         ...
  91         trap[N-1] rd rs1 rs2: xcmd[N-1] rd rs1 rs2
  92     }
  93
  94 each of the xcmd instructions shall trap to one level higher.
  95
  96 At privilege level machine mode each trap command has unspecified behaviour, but in debug mode
  97 should cause an exception to a debug environment.
  98
  99 For lun == 1, at all privilege levels
 100
 101     org.RiscV:Fallback:ReturnZero{
 102         uuid: 1
 103         return_zero0 rd rs1 rs2: xcmd0 rd rs1 rs2
 104         ...
 105         return_zero[N-1] rd rs1 rs2: xcmd[N-1] rd rs1 rs2
 106     }
 107
 108 each return_zero command shall return 0 in rd.
 109
 110 For lun == 2, at all privilege levels
 111
 112     org.RiscV:Fallback:ReturnMinusOne{
 113         uuid: 2
 114         return_minusone0 rd rs1 rs2: xcmd0 rd rs1 rs2
 115         ...
 116         return_minusone[N-1] rd rs1 rs2: xcmd[N-1] rd rs1 rs2
 117     }
 118
 119 each return_minusone shall return -1.
 120
 121 ---
 122
 123 Remark:
 124 Quite possibly even glorified standard assembler macros are overkill and it is easier to just use defines or ordinary macro's with long names. E.g. writing
 125
 126     #define org_tinker_tinker__RocknRoll__uuid 0xABCDE
 127     #define org_tinker_tinker__RocknRoll__rock(rd, rs1, rs2) xcmd0 rd, rs1, rs2
 128     #define org_tinker_tinker__RocknRoll__roll(rd, rs1, rs2) xcmd1 rd, rs1, rs2
 129
 130 allows the same sequence to be written as
 131
 132     lui   rd org_tinker_tinker__RocknRoll__uuid
 133     xext  rd rs1
 134     org_tinker_tinker__RocknRoll__rock(rd, rd, rs2)
 135
 136 Readability of assembler is no big deal for a compiler, but people are supposed to _document_ the interface and its semantics. In particular specifying the semantics of the xintf in same way as the semantics of the cpu should allow formal verification.
 137
 138 ==Implications for the RiscV ecosystem ==
 139
 140
 141 The proposal allows independent groups to define one or more extension
 142 interfaces of (slightly crippled) R-type instructions implemented by an
 143 extension device. Such an extension device would be an native but non standard
 144 extension of the CPU, an IP tile or a closely coupled external chip and would
 145 be configured at manufacturing time or bootup of the CPU.
 146
 147 Having a standardised overloadable interface simply avoids much of the
 148 need for isa extensions for hardware with non standard interfaces and
 149 semantics. This is analogous to the way that the standardised overloadable
 150 ioctl interface of the kernel almost completely avoids the need for
 151 extending the kernel with syscalls for the myriad of hardware devices
 152 with their specific interfaces and semantics.
 153
 154 The expanded flexibility comes at the cost: the standard can specify the
 155 semantics of the delegation mechanism and the interfacing with the rest
 156 of the cpu, but the actual semantics of the overloaded instructions can
 157 only be defined by the designer of the interface. Likewise, a device
 158 can be conforming as far as delegation and interaction with the CPU
 159 is concerned, but whether the hardware is conforming to the semantics
 160 of the interface is outside the scope of spec. Being able to specify
 161 that semantics using the methods used for RV itself is clearly very
 162 valuable. One impetus for doing that is using it for purposes of its own,
 163 effectively freeing opcode space for other purposes. Also, some interfaces
 164 may become de facto or de jure standards themselves, necessitating
 165 hardware to implement competing interfaces. I.e., facilitating a free
 166 for all, may lead to standards proliferation. C'est la vie.
 167
 168 The only "ISA-collisions" that can still occur are in the 20 bit (~10^6)
 169 interface identifier space, with 12 more bits to identify a device on
 170 a hart that implements the interface. One suggestion is setting aside
 171 2^19 id's that are handed out for a small fee by a central (automated)
 172 registration (making sure the space is not just claimed), while the
 173 remaining 2^19 are used as a good hash on a long, plausibly globally
 174 unique human readable interface name. This gives implementors the choice
 175 between a guaranteed private identifier paying a fee, or relying on low
 176 probabilities. On RV64 the UUID can also be extended to 52 bits (> 10^15).
 177
 178
 179 ==== Description of the extension as C functions.==
 180
 181      /* register format of rs1 for xext instructions */
 182      typedef struct uuid_device{
 183        long dev:12;
 184        long uuid: 8*sizeof(long) - 12;
 185      } uuid_device_t
 186
 187      /* register format for rd of xext and rs1 of xcmd instructions, packs lun and data */
 188      typedef struct lun_data{
 189        long lun:12;
 190        long data: 8*sizeof(long) - 12;
 191      } lun_data_t
 192
 193      /* proposed R-type instructions
 194      xext  rd rs1 rs2
 195      xcmd0 rd rs1 rs2
 196      xcmd1 rd rs1 rs2
 197      ...
 198      xcmd7 rd rs1 rs2
 199      */
 200
 201      lun_data_t xext(uuid_dev_t rs1, long rs2);
 202      long xcmd0(lun_data_t rs1, long rs2);
 203      long xcmd1(lun_data_t rs1, long rs2);
 204      ...
 205      long xcmd<N>(lun_data_t rs1, long rs2);
 206
 207      /* hardware interface presented by an implementing device. */
 208      typedef
 209      long device_fn(unsigned short subdevice_xcmd, lun_data_t rs1, long rs2);
 210
 211      /* cpu internal datatypes */
 212
 213      enum privilege = {user = 0b0001, super = 0b0010, hyper = 0b0100, mach = 0b1000};
 214
 215      /* cpu internal, does what is on the label */
 216      static
 217      enum privilege cpu__current_privilege_level()
 218
 219      typedef
 220      struct lun{
 221        unsigned short id:12
 222      } lun_t;
 223
 224      struct uuid_device_priv2lun{
 225        struct{
 226           uuid_dev_t    uuid_dev;
 227           enum privilege reqpriv;
 228        };
 229        lun_t lun;
 230      };
 231
 232      struct device_subdevice{
 233        device_fn* device_addr;
 234        unsigned short subdeviceId:12;
 235      };
 236
 237      struct lun_priv2device_subdevice{
 238        struct{
 239            lun_t lun;
 240            enum privilege reqpriv
 241        }
 242        struct device_subdevice devAddr_subdevId;
 243      }
 244
 245      static
 246      struct uuid_device_priv2lun cpu__lun_map[];
 247
 248      /*
 249        map (UUID, device, privilege) to a 12 bit lun,
 250        return (lun_t){0} on unknown  (at acces level)
 251
 252        does associative memory lookup and tests privilege.
 253     */
 254     static
 255     lun_t cpu__lookup_lun(const struct uuid_device_priv2lun* lun_map, uuid_dev_t uuid_dev, enum privilege priv);
 256
 257
 258
 259      lun_data_t xext(uuid_dev_t rs1, long rs2)
 260      {
 261         lun_t lun = cpu__lookup_lun(lun_map, rs1, current_privilege_level());
 262
 263         return (lun_data_t){.lun = lun.id, .data = rs2 % (1<< (8*sizeof(long) - 12))}
 264      }
 265
 266
 267      struct lun_priv2device_subdevice cpu__device_subdevice_map[];
 268
 269      /* map (lun, priv)  to struct device_subdevice pair.
 270         For lun = 0, or unknown (lun, priv) pair, returns (struct device_subdevice){NULL,0}
 271      */
 272      static
 273      device_subdevice_t cpu__lookup_device_subdevice(const struct lun_priv2device_subdevice_map* dev_subdev_map,
 274                                                      lun_t lun, enum privileges priv);
 275
 276
 277
 278      /* functional description of the delegating xcmd0 .. xcmd7 instructions */
 279      template<k = 0..N-1>   //pretend this is C
 280      long xcmd<k>(lun_data_t rs1, long rs2)
 281      {
 282          struct device_subdevice dev_subdev = cpu__lookup_device_subdevice(device_subdevice_map, rs1.lun, current_privilege());
 283          return dev_subdev.devAddr(dev_subdev.subdevId | k >> 12 , rs1, rs2);
 284      }
 285
 286      /*Fallback interfaces*/
 287      #define org_RiscV__Fallback__Trap__uuid 0
 288      #define org_RiscV__Fallback__ReturnZero__uuid 1
 289      #define org_RiscV__Fallback__ReturnMinusOne__uuid 2
 290
 291      /* fallback device */
 292      static
 293      long cpu__falback(short subdevice_xcmd,  lun_data_t rs1, long rs2)
 294      {
 295         switch(subdevice_xcmd % (1 << 12) ){
 296         case 0  /* org.RiscV:Trap */:         trap_to(cpu__next_higher_privilege_level());
 297         case 1  /* org.RiscV:ReturnZero */:   return 0;
 298         case 2  /* org.RiscV:ReturnMinus1 */: return -1
 299         case 3  /* org.RiscV:Trap Machinelevel */: printf("something is rotten in machinemode: unknown xintf device"); return 31415926;
 300         default: trap("hardware configuration error");
 301      }
 302
 303 Example:
 304
 305
 306
 307      #define com_bigbucks__Frobate__uuid 0xABCDE
 308      #define org_tinker_tinker__RocknRoll__uuid 0x12345
 309      #define org_tinker_tinker__Jazz__uuid 0xD0B0D
 310      /*
 311      com.bigbucks:Frobate{
 312          uuid: com_bigbucks__Frobate__uuid
 313          frobate rd rs1 rs2 : cmd0 rd rs1 rs2
 314          foo     rd rs1 rs2 : cmd1 rd rs1 rs2
 315          bar     rd rs1 rs2 : cmd1 rd rs1 rs2
 316      }
 317      */
 318      org.tinker.tinker:RocknRoll{
 319          uuid: org_tinker_tinker__RocknRoll__uuid
 320          rock rd rs1 rs2: cmd0 rd rs1 rs2
 321          roll rd rs1 rs2: cmd1 rd rs1 rs2
 322      }
 323
 324      long com_bigbucks__device1(short  subdevice_xcmd, lun_data_t rs1, long rs2)
 325      {
 326         switch(subdevice_xcmd) {
 327         case 0 | 0 << 12  /* com.bigbucks:Frobate:frobate */     : return device1_frobate(rs1, rs2);
 328         case 42| 0 << 12  /* com.bigbucks:FrobateMach:frobate    : return device1_frobate_machine_level(rs1, rs2);
 329         case 0 | 1 << 12  /* com.bigbucks:Frobate:foo */         : return device1_foo(rs1, rs2);
 330         case 0 | 2 << 12  /* com.bigbucks:Frobate:bar */         : return device1_bar(rs1, rs2);
 331         case 1 | 0 << 12  /* org.tinker.tinker:RocknRoll:rock */ : return device1_rock(rs1, rs2);
 332         case 1 | 1 << 12  /* org.tinker.tinker:RocknRoll:roll */ : return device1_roll(rs1, rs2);
 333         default: trap(“hardware configuration error”);
 334         }
 335      }
 336
 337      /*
 338      org.tinker.tinker:Jazz{
 339        uuid: org_tinker_tinker__Jazz__uuid
 340        boogy rd rs1 rs2: cmd0 rd rs1 rs2
 341      }
 342      */
 343
 344      long org_tinker_tinker__device2(short subdevice_xcmd,  lun_data_t rs1, long rs2)
 345      {
 346         switch(dev_cmd.interfId){
 347         case 0  | 0 << 12 /* com.bigbucks:Frobate:frobate */: return device2_frobate(rs1, rs2);
 348         case 0  | 1 << 12 /* com.bigbucks:Frobate:foo */    : return device2_foo(rs1, rs2);
 349         case 0  | 2 << 12 /* com.bigbucks:Frobate:bar */    : return device2_foo(rs1, rs2);
 350         case 1  | 0 << 12 /* org_tinker_tinker:Jazz:boogy */: return device2_boogy(rs1, rs2);
 351         default: trap(“hardware configuration error”);
 352         }
 353      }
 354
 355
 356      /* struct uuid_dev2lun_map[] */
 357      lun_map = {
 358             {{.uuid_devId = {org_RiscV__Fallback__Trap__uuid , 0},          .priv = user},  .lun =  0},
 359             {{.uuid_devId = {org_RiscV__Fallback__Trap__uuid , 0},          .priv = super}, .lun =  0},
 360             {{.uuid_devId = {org_RiscV__Fallback__Trap__uuid , 0},          .priv = hyper}, .lun =  0},
 361             {{.uuid_devId = {org_RiscV__Fallback__Trap__uuid , 0},          .priv = mach},  .lun =  0},
 362             {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = user},  .lun =  1},
 363             {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = super}, .lun =  1},
 364             {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = hyper}, .lun =  1},
 365             {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = mach}   .lun =  1},
 366             {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = user},  .lun =  2},
 367             {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = super}, .lun =  2},
 368             {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = hyper}, .lun =  2},
 369             {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = mach},  .lun =  2},
 370             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = user}   .lun = 32},  //32 sic!
 371             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 1},               .priv = super}  .lun = 32},
 372             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 1},               .priv = hyper}  .lun = 32},
 373             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 1},               .priv = mach}   .lun = 32},
 374             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = super}  .lun = 34},  //34 sic!
 375             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = hyper}  .lun = 34},
 376             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = mach}   .lun = 34},
 377             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = user}   .lun = 33},  //33 sic!
 378             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = super}  .lun = 33},
 379             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = hyper}  .lun = 33},
 380             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = super}, .lun = 35},
 381             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = hyper}, .lun = 35},
 382        }
 383
 384      /* struct lun2dev_subdevice_map[] */
 385         dev_subdevice_map = {
 386              {{.lun = 0, .priv = user},  .devAddr_interfId = {fallback,    0 /* Trap  */}},
 387              {{.lun = 0, .priv = super}, .devAddr_interfId = {fallback,    0 /* Trap  */}},
 388              {{.lun = 0, .priv = hyper}, .devAddr_interfId = {fallback,    0 /* Trap  */}},
 389              {{.lun = 0, .priv = mach},  .devAddr_interfId = {fallback,    3 /* Trap  */}},
 390              {{.lun = 1, .priv = user},  .devAddr_interfId = {fallback,    1 /* ReturnZero  */}},
 391              {{.lun = 1, .priv = super}, .devAddr_interfId = {fallback,    1 /* ReturnZero  */}},
 392              {{.lun = 1, .priv = hyper}, .devAddr_interfId = {fallback,    1 /* ReturnZero  */}},
 393              {{.lun = 1, .priv = mach},  .devAddr_interfId = {fallback,    1 /* ReturnZero  */}},
 394              {{.lun = 2, .priv = user},  .devAddr_interfId = {fallback,    2 /* ReturnMinusOne*/}},
 395              {{.lun = 2, .priv = super}, .devAddr_interfId = {fallback,    2 /* ReturnMinusOne*/}},
 396              {{.lun = 2, .priv = hyper}, .devAddr_interfId = {fallback,    2 /* ReturnMinusOne*/}},
 397              {{.lun = 2, .priv = mach},  .devAddr_interfId = {fallback,    2 /* ReturnMinusOne*/}},
 398      //       .lun = 3 .. 7  reserved for other fallback RV interfaces
 399      //       .lun = 8 .. 30 reserved as error numbers, c.li t1 31; bltu rd t1 L_fail tests errors
 400      //      .lun = 31  reserved out of caution
 401              {{.lun = 32, .priv = user},  .devAddr_interfId = {device1, 0 /* Frobate  interface */}},
 402              {{.lun = 32, .priv = super}, .devAddr_interfId = {device1, 0 /* Frobate  interface */}},
 403              {{.lun = 32, .priv = hyper}, .devAddr_interfId = {device1, 0 /* Frobate  interface */}},
 404              {{.lun = 32, .priv = mach},  .devAddr_interfId = {device1,64 /* Frobate  machine level interface */}},
 405              {{.lun = 33, .priv = user},  .devAddr_InterfId = {device1, 1 /* RocknRoll interface */}},
 406              {{.lun = 33, .priv = super}, .devAddr_InterfId = {device1, 1 /* RocknRoll interface */}},
 407              {{.lun = 33, .priv = hyper}, .devAddr_InterfId = {device1, 1 /* RocknRoll interface */}},
 408              {{.lun = 34, .priv = super}, .devAddr_interfId = {device2, 0 /* Frobate interface */}},
 409              {{.lun = 34, .priv = hyper}, .devAddr_interfId = {device2, 0 /* Frobate interface */}},
 410              {{.lun = 34, .priv = mach},  .devAddr_interfId = {device2, 0 /* Frobate interface */}},
 411              {{.lun = 35, .priv = super}, .devAddr_interfId = {device2, 1 /* Jazz interface */}},
 412              {{.lun = 35, .priv = hyper}, .devAddr_interfId = {device2, 1 /* Jazz interface */}},
 413          }