pluggable_extensions.mdwn

   1 # pluggable extensions
   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> rd, rs1, rs2, that are intended to be used as "overloadable" (slightly crippled) R-type instructions for independently developed extensions 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 The input value of an xcmd instruction in rs2 is arbitrary. The content of the first input rs1, however, is divided in a 12bit "logical unit" (lun)  together with xlen - 12 bits of additional data.
  10 The lun bits in rs1, determines a specific (sub)device, and the CPU routes the command to this device with rs1 and rs2 as input, and rd as output. Effectively, the xcmd0, ... xcmd7 instructions are "virtual method" opcodes, overloaded for different extension (sub)devices.
  11
  12 The specific value of the lun is supposed to be convenient for the cpu and is thus unstandardised. Portable software therefore constructs the lun, with a further R-type instruction xext. It takes a 20 bit universally unique identifier (UUID) that identifies  an interface with upto N R-type instructions with the signature of xcmd. An optional sequence number identifies a specific enumerated device on the cpu that implements the interface as a subdevice. For convenience, xext also or's bits rs2[0..XLEN-12]. If the UUID is not recognised 0 is returned. , but implemented by the extension (sub)device. Note that this scheme gives an easy work around the restriction on N (e.g. 8 ) commands: an implementing device can simply implement several interfaces as routable subdevices, indeed is expected to do so.
  13
  14 The net effect is that a sequence like
  15
  16     //fake UUID
  17     lui   rd 0xEDCBA
  18     xext  rd rd rs1
  19     xcmd0 rd rd rs2
  20
  21 acts like a single namespaced instruction cmd0_EDCBA 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
  22 additional performance price compared to a "native" instruction. This should, however, be an acceptable tradeoff in many cases.
  23
  24
  25 Programatically the instructions in the interface are just a set of glorified assembler macros
  26
  27      org.tinker.tinker:RocknRoll{
  28         uuid : 0xABCDE
  29         rock rd rs1 rs2 : xcmd0 rd rs1 rs2
  30         roll rd rs1 rs2 : xcmd1 rd rs1 rs2
  31      }
  32
  33 so that the above sequence is more clearly written as
  34
  35     import(org.tinker.tinker:RocknRoll)
  36
  37     lui rd org.tinker.tinker:RocknRoll:uuid
  38     xext rd rd rs1
  39     org.tinker.tinker:RocknRoll:rock rd rd rs2
  40
  41 (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
  42
  43     #define org_tinker_tinker__RocknRoll__uuid 0xABCDE
  44     #define org_tinker_tinker__RocknRoll__rock(rd, rs1, rs2) xcmd0 rd, rs1, rs2
  45     #define org_tinker_tinker__RocknRoll__roll(rd, rs1, rs2) xcmd1 rd, rs1, rs2
  46
  47 allows the same sequence to be written as
  48
  49     lui   rd org_tinker_tinker__RocknRoll__uuid
  50     xext  rd rs1
  51     org_tinker_tinker__RocknRoll__rock(rd, rd, rs2)
  52
  53 Readability of assembler is no big deal for a compiler, but people are supposed to _document_ the interface and its semantics. In particular a semantics specified like the semantics of the cpu would be most welcome.)
  54
  55
  56 If several instructions of the same interface are used, one can also use instruction sequences like
  57
  58     lui   t1 org_tinker_tinker__RocknRoll_uuid
  59     xext  t1 zero
  60     xcmd0 a5, t1, a0  // org_tinker_tinker__RocknRoll__rock(a5, t1, a0)
  61     xcmd1 t2, t1, a1  // org_tinker_tinker__RocknRoll__roll(t2, t1, a5)
  62     xcmd0 a0, t1, t2  // org_tinker_tinker__RocknRoll__rock(a0, t1, t2)
  63
  64 This amortises the cost of the xext instruction.
  65
  66 ==Implications for the RiscV ecosystem ==
  67
  68
  69 The proposal allows independent groups to define one or more extension
  70 interfaces of (slightly crippled) R-type instructions implemented by an
  71 extension device. Such an extension device would be an native but non standard
  72 extension of the CPU, an IP tile or a closely coupled external chip and would
  73 be configured at manufacturing time or bootup of the CPU.
  74
  75 Having a standardised overloadable interface simply avoids much of the
  76 need for isa extensions for hardware with non standard interfaces and
  77 semantics. This is analogous to the way that the standardised overloadable
  78 ioctl interface of the kernel almost completely avoids the need for
  79 extending the kernel with syscalls for the myriad of hardware devices
  80 with their specific interfaces and semantics.
  81
  82 The expanded flexibility comes at the cost: the standard can specify the
  83 semantics of the delegation mechanism and the interfacing with the rest
  84 of the cpu, but the actual semantics of the overloaded instructions can
  85 only be defined by the designer of the interface. Likewise, a device
  86 can be conforming as far as delegation and interaction with the CPU
  87 is concerned, but whether the hardware is conforming to the semantics
  88 of the interface is outside the scope of spec. Being able to specify
  89 that semantics using the methods used for RV itself is clearly very
  90 valuable. One impetus for doing that is using it for purposes of its own,
  91 effectively freeing opcode space for other purposes. Also, some interfaces
  92 may become de facto or de jure standards themselves, necessitating
  93 hardware to implement competing interfaces. I.e., facilitating a free
  94 for all, may lead to standards proliferation. C'est la vie.
  95
  96 The only "ISA-collisions" that can still occur are in the 20 bit (~10^6)
  97 interface identifier space, with 12 more bits to identify a device on
  98 a hart that implements the interface. One suggestion is setting aside
  99 2^19 id's that are handed out for a small fee by a central (automated)
 100 registration (making sure the space is not just claimed), while the
 101 remaining 2^19 are used as a good hash on a long, plausibly globally
 102 unique human readable interface name. This gives implementors the choice
 103 between a guaranteed private identifier paying a fee, or relying on low
 104 probabilities. On RV64 the UUID can also be extended to 52 bits (> 10^15).
 105
 106
 107 ==== Description of the extension as C functions.==
 108
 109      /* register format of rs1 for xext instructions */
 110      typedef struct uuid_device{
 111        long dev:12;
 112        long uuid: 8*sizeof(long) - 12;
 113      } uuid_device_t
 114
 115      /* register format for rd of xext and rs1  for xcmd instructions, packs lun and data */
 116      typedef struct lun_data{
 117        long lun:12;
 118        long data: 8*sizeof(long) - 12;
 119      } lun_data_t
 120
 121      /* proposed R-type instructions
 122      xext  rd rs1 rs2
 123      xcmd0 rd rs1 rs2
 124      xcmd1 rd rs1 rs2
 125      ...
 126      xcmd7 rd rs1 rs2
 127      */
 128
 129      lun_data_t xext(uuid_dev_t rs1, long rs2);
 130      long xcmd0(lun_data_t rs1, long rs2);
 131      long xcmd1(lun_data_t rs1, long rs2);
 132      ...
 133      long xcmd<N>(lun_data_t rs1, long rs2);
 134
 135      /* hardware interface presented by an implementing device. */
 136      typedef
 137      long device_fn(unsigned short subdevice_xcmd, lun_data_t rs1, long rs2);
 138
 139      /* cpu internal datatypes */
 140
 141      enum privilege = {user = 0b0001, super = 0b0010, hyper = 0b0100, mach = 0b1000};
 142
 143      /* cpu internal, does what is on the label */
 144      static
 145      enum privilege cpu__current_privilege_level()
 146
 147      typedef
 148      struct lun{
 149        unsigned short id:12
 150      } lun_t;
 151
 152      struct uuid_device_priv2lun{
 153        struct{
 154           uuid_dev_t    uuid_dev;
 155           enum privilege reqpriv;
 156        };
 157        lun_t lun;
 158      };
 159
 160      struct device_subdevice{
 161        device_fn* device_addr;
 162        unsigned short subdeviceId:12;
 163      };
 164
 165      struct lun_priv2device_subdevice{
 166        struct{
 167            lun_t lun;
 168            enum privilege reqpriv
 169        }
 170        struct device_subdevice devAddr_subdevId;
 171      }
 172
 173      static
 174      struct uuid_device_priv2lun cpu__lun_map[];
 175
 176      /*
 177        map (UUID, device, privilege) to a 12 bit lun,
 178        return (lun_t){0} on unknown  (at acces level)
 179
 180        does associative memory lookup and tests privilege.
 181     */
 182     static
 183     lun_t cpu__lookup_lun(const struct uuid_device_priv2lun* lun_map, uuid_dev_t uuid_dev, enum privilege priv);
 184
 185
 186
 187      lun_data_t xext(uuid_dev_t rs1, long rs2)
 188      {
 189         lun_t lun = cpu__lookup_lun(lun_map, rs1, current_privilege_level());
 190
 191         return (lun_data_t){.lun = lun.id, .data = rs2 % (1<< (8*sizeof(long) - 12))}
 192      }
 193
 194
 195
 196
 197      struct lun_priv2device_subdevice cpu__device_subdevice_map[];
 198
 199      /* map (lun, priv)  to struct device_subdevice pair.
 200         For lun = 0, or unknown (lun, priv) pair, returns (struct device_subdevice){NULL,0}
 201      */
 202      static
 203      device_subdevice_t cpu__lookup_device_subdevice(const struct lun_priv2device_subdevice_map* dev_subdev_map,
 204                                                      lun_t lun, enum privileges priv);
 205
 206      /* functional description of the delegating xcmd0 .. xcmd7 instructions */
 207      template<k = 0..N-1> //pretend this is C
 208      long xcmd<k>(lun_data_t rs1, long rs2)
 209      {
 210          struct device_subdevice dev_subdev = cpu__lookup_device_subdevice(device_subdevice_map, rs1.lun, current_privilege());
 211          if(dev_subdev.devAddr == NULL)
 212             trap(“Illegal instruction”);
 213
 214          return dev_subdev.devAddr(dev_subdev.subdevId | k << 12, rs1, rs2);
 215      }
 216
 217
 218
 219 Example:
 220
 221      #define com_bigbucks__Frobate__uuid 0xABCDE
 222      #define org_tinker_tinker__RocknRoll__uuid 0x12345
 223      #define org_tinker_tinker__Jazz__uuid 0xD0B0D
 224      /*
 225      com.bigbucks:Frobate{
 226          uuid: com_bigbucks__Frobate__uuid
 227          frobate rd rs1 rs2 : cmd0 rd rs1 rs2
 228          foo     rd rs1 rs2 : cmd1 rd rs1 rs2
 229          bar     rd rs1 rs2 : cmd1 rd rs1 rs2
 230      }
 231      */
 232      org.tinker.tinker:RocknRoll{
 233          uuid: org_tinker_tinker__RocknRoll__uuid
 234          rock rd rs1 rs2: cmd0 rd rs1 rs2
 235          roll rd rs1 rs2: cmd1 rd rs1 rs2
 236      }
 237
 238      long com_bigbucks__device1(short  subdevice_xcmd, lun_data_t rs1, long rs2)
 239      {
 240         switch(subdevice_xcmd) {
 241         case 0 | 0 << 12  /* com.bigbucks:Frobate:frobate */     : return device1_frobate(rs1, rs2);
 242         case 42| 0 << 12  /* com.bigbucks:FrobateMach:frobate    : return device1_frobate_machine_level(rs1, rs2);
 243         case 0 | 1 << 12  /* com.bigbucks:Frobate:foo */         : return device1_foo(rs1, rs2);
 244         case 0 | 2 << 12  /* com.bigbucks:Frobate:bar */         : return device1_bar(rs1, rs2);
 245         case 1 | 0 << 12  /* org.tinker.tinker:RocknRoll:rock */ : return device1_rock(rs1, rs2);
 246         case 1 | 1 << 12  /* org.tinker.tinker:RocknRoll:roll */ : return device1_roll(rs1, rs2);
 247         default: trap(“hardware configuration error”);
 248         }
 249      }
 250
 251      /*
 252      org.tinker.tinker:Jazz{
 253        uuid: org_tinker_tinker__Jazz__uuid
 254        boogy rd rs1 rs2: cmd0 rd rs1 rs2
 255      }
 256      */
 257
 258      long org_tinker_tinker__device2(short subdevice_xcmd,  lun_data_t rs1, long rs2)
 259      {
 260         switch(dev_cmd.interfId){
 261         case 0  | 0 << 12 /* com.bigbucks:Frobate:frobate */: return device2_frobate(rs1, rs2);
 262         case 0  | 1 << 12 /* com.bigbucks:Frobate:foo */    : return device2_foo(rs1, rs2);
 263         case 0  | 2 << 12 /* com.bigbucks:Frobate:bar */    : return device2_foo(rs1, rs2);
 264         case 1  | 0 << 12 /* org_tinker_tinker:Jazz:boogy */: return device2_boogy(rs1, rs2);
 265         default: trap(“hardware configuration error”);
 266         }
 267      }
 268
 269         /* struct uuid_dev2lun_map[] */
 270         lun_map = {
 271             {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = user},  .lun =  1},
 272             {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = super}, .lun =  1},
 273             {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = hyper}, .lun =  1},
 274             {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = mach}   .lun =  1},
 275             {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = user},  .lun =  2},
 276             {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = super}, .lun =  2},
 277             {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = hyper}, .lun =  2},
 278             {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = mach},  .lun =  2},
 279             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = user}   .lun = 32},  //32 sic!
 280             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 1},               .priv = super}  .lun = 32},
 281             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 1},               .priv = hyper}  .lun = 32},
 282             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 1},               .priv = mach}   .lun = 32},
 283             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = super}  .lun = 34},  //34 sic!
 284             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = hyper}  .lun = 34},
 285             {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = mach}   .lun = 34},
 286             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = user}   .lun = 33},  //33 sic!
 287             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = super}  .lun = 33},
 288             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = hyper}  .lun = 33},
 289             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = super}, .lun = 35},
 290             {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = hyper}, .lun = 35},
 291        }
 292
 293      /* struct lun2dev_subdevice_map[] */
 294         dev_subdevice_map = {
 295       //     {.lun = 0,   error and falls back to trapping xcmd
 296              {{.lun = 1, .priv = user},  .devAddr_interfId = {fallback,    0 /* ReturnZero  */}},
 297              {{.lun = 1, .priv = super}, .devAddr_interfId = {fallback,    0 /* ReturnZero  */}},
 298              {{.lun = 1, .priv = hyper}, .devAddr_interfId = {fallback,    0 /* ReturnZero  */}},
 299              {{.lun = 1, .priv = mach},  .devAddr_interfId = {fallback,    0 /* ReturnZero  */}},
 300              {{.lun = 2, .priv = user},  .devAddr_interfId = {fallback,    1 /* ReturnMinusOne*/}},
 301              {{.lun = 2, .priv = super}, .devAddr_interfId = {fallback,    1 /* ReturnMinusOne*/}},
 302              {{.lun = 2, .priv = hyper}, .devAddr_interfId = {fallback,    1 /* ReturnMinusOne*/}},
 303              {{.lun = 2, .priv = mach},  .devAddr_interfId = {fallback,    1 /* ReturnMinusOne*/}},
 304      //       .lun = 3 .. 7  reserved for other fallback RV interfaces
 305      //       .lun = 8 .. 30 reserved as error numbers, c.li t1 31; bltu rd t1 L_fail tests errors
 306      //      .lun = 31  reserved out of caution
 307              {{.lun = 32, .priv = user},  .devAddr_interfId = {device1, 0 /* Frobate  interface */}},
 308              {{.lun = 32, .priv = super}, .devAddr_interfId = {device1, 0 /* Frobate  interface */}},
 309              {{.lun = 32, .priv = hyper}, .devAddr_interfId = {device1, 0 /* Frobate  interface */}},
 310              {{.lun = 32, .priv = mach},  .devAddr_interfId = {device1,64 /* Frobate  machine level interface */}},
 311              {{.lun = 33, .priv = user},  .devAddr_InterfId = {device1, 1 /* RocknRoll interface */}},
 312              {{.lun = 33, .priv = super}, .devAddr_InterfId = {device1, 1 /* RocknRoll interface */}},
 313              {{.lun = 33, .priv = hyper}, .devAddr_InterfId = {device1, 1 /* RocknRoll interface */}},
 314              {{.lun = 34, .priv = super}, .devAddr_interfId = {device2, 0 /* Frobate interface */}},
 315              {{.lun = 34, .priv = hyper}, .devAddr_interfId = {device2, 0 /* Frobate interface */}},
 316              {{.lun = 34, .priv = mach},  .devAddr_interfId = {device2, 0 /* Frobate interface */}},
 317              {{.lun = 35, .priv = super}, .devAddr_interfId = {device2, 1 /* Jazz interface */}},
 318              {{.lun = 35, .priv = hyper}, .devAddr_interfId = {device2, 1 /* Jazz interface */}},
 319          }