pluggable extensions

This proposal adds a standardised extension instructions to the RV instruction set by introducing a fixed small number N (e.g. N = 8) of R-type opcodes xcmd0 rd, rs1, rs2, .. , xcmd 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.

Tl;DR see below for a C description of how this is supposed to work.

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. 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.

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.

The net effect is that a sequence like

//fake UUID
lui   rd 0xEDCBA
xext  rd rd rs1
xcmd0 rd rd rs2 

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 additional performance price compared to a "native" instruction. This should, however, be an acceptable tradeoff in many cases.

Programatically the instructions in the interface are just a set of glorified assembler macros

 org.tinker.tinker:RocknRoll{
    uuid : 0xABCDE
    rock rd rs1 rs2 : xcmd0 rd rs1 rs2
    roll rd rs1 rs2 : xcmd1 rd rs1 rs2
 }

so that the above sequence is more clearly written as

import(org.tinker.tinker:RocknRoll)

lui rd org.tinker.tinker:RocknRoll:uuid
xext rd rd rs1
org.tinker.tinker:RocknRoll:rock rd rd rs2

(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

#define org_tinker_tinker__RocknRoll__uuid 0xABCDE 
#define org_tinker_tinker__RocknRoll__rock(rd, rs1, rs2) xcmd0 rd, rs1, rs2
#define org_tinker_tinker__RocknRoll__roll(rd, rs1, rs2) xcmd1 rd, rs1, rs2

allows the same sequence to be written as

lui   rd org_tinker_tinker__RocknRoll__uuid 
xext  rd rs1
org_tinker_tinker__RocknRoll__rock(rd, rd, rs2)

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.)

If several instructions of the same interface are used, one can also use instruction sequences like

lui   t1 org_tinker_tinker__RocknRoll_uuid
xext  t1 zero
xcmd0 a5, t1, a0  // org_tinker_tinker__RocknRoll__rock(a5, t1, a0) 
xcmd1 t2, t1, a1  // org_tinker_tinker__RocknRoll__roll(t2, t1, a5)
xcmd0 a0, t1, t2  // org_tinker_tinker__RocknRoll__rock(a0, t1, t2)

This amortises the cost of the xext instruction.

==Implications for the RiscV ecosystem ==

The proposal allows independent groups to define one or more extension interfaces of (slightly crippled) R-type instructions implemented by an extension device. Such an extension device would be an native but non standard extension of the CPU, an IP tile or a closely coupled external chip and would be configured at manufacturing time or bootup of the CPU.

Having a standardised overloadable interface simply avoids much of the need for isa extensions for hardware with non standard interfaces and semantics. This is analogous to the way that the standardised overloadable ioctl interface of the kernel almost completely avoids the need for extending the kernel with syscalls for the myriad of hardware devices with their specific interfaces and semantics.

The expanded flexibility comes at the cost: the standard can specify the semantics of the delegation mechanism and the interfacing with the rest of the cpu, but the actual semantics of the overloaded instructions can only be defined by the designer of the interface. Likewise, a device can be conforming as far as delegation and interaction with the CPU is concerned, but whether the hardware is conforming to the semantics of the interface is outside the scope of spec. Being able to specify that semantics using the methods used for RV itself is clearly very valuable. One impetus for doing that is using it for purposes of its own, effectively freeing opcode space for other purposes. Also, some interfaces may become de facto or de jure standards themselves, necessitating hardware to implement competing interfaces. I.e., facilitating a free for all, may lead to standards proliferation. C'est la vie.

The only "ISA-collisions" that can still occur are in the 20 bit (~106) interface identifier space, with 12 more bits to identify a device on a hart that implements the interface. One suggestion is setting aside 219 id's that are handed out for a small fee by a central (automated) registration (making sure the space is not just claimed), while the remaining 219 are used as a good hash on a long, plausibly globally unique human readable interface name. This gives implementors the choice between a guaranteed private identifier paying a fee, or relying on low probabilities. On RV64 the UUID can also be extended to 52 bits (> 1015).

==== Description of the extension as C functions.==

 /* register format of rs1 for xext instructions */
 typedef struct uuid_device{
   long dev:12;
   long uuid: 8*sizeof(long) - 12;
 } uuid_device_t

 /* register format for rd of xext and rs1  for xcmd instructions, packs lun and data */
 typedef struct lun_data{
   long lun:12;
   long data: 8*sizeof(long) - 12;
 } lun_data_t

 /* proposed R-type instructions 
 xext  rd rs1 rs2
 xcmd0 rd rs1 rs2
 xcmd1 rd rs1 rs2
 ...
 xcmd7 rd rs1 rs2
 */

 lun_data_t xext(uuid_dev_t rs1, long rs2);
 long xcmd0(lun_data_t rs1, long rs2);
 long xcmd1(lun_data_t rs1, long rs2);
 ...
 long xcmd<N>(lun_data_t rs1, long rs2);

 /* hardware interface presented by an implementing device. */
 typedef
 long device_fn(unsigned short subdevice_xcmd, lun_data_t rs1, long rs2);

 /* cpu internal datatypes */

 enum privilege = {user = 0b0001, super = 0b0010, hyper = 0b0100, mach = 0b1000};

 /* cpu internal, does what is on the label */
 static
 enum privilege cpu__current_privilege_level()

 typedef 
 struct lun{
   unsigned short id:12
 } lun_t;

 struct uuid_device_priv2lun{
   struct{
      uuid_dev_t    uuid_dev;
      enum privilege reqpriv;
   };
   lun_t lun;
 };

 struct device_subdevice{
   device_fn* device_addr;
   unsigned short subdeviceId:12;
 };      

 struct lun_priv2device_subdevice{
   struct{
       lun_t lun;
       enum privilege reqpriv
   }
   struct device_subdevice devAddr_subdevId;
 }

 static 
 struct uuid_device_priv2lun cpu__lun_map[];

 /* 
   map (UUID, device, privilege) to a 12 bit lun, 
   return (lun_t){0} on unknown  (at acces level)

   does associative memory lookup and tests privilege.
*/
static
lun_t cpu__lookup_lun(const struct uuid_device_priv2lun* lun_map, uuid_dev_t uuid_dev, enum privilege priv);



 lun_data_t xext(uuid_dev_t rs1, long rs2)
 {
    lun_t lun = cpu__lookup_lun(lun_map, rs1, current_privilege_level());

    return (lun_data_t){.lun = lun.id, .data = rs2 % (1<< (8*sizeof(long) - 12))}
 }




 struct lun_priv2device_subdevice cpu__device_subdevice_map[];

 /* map (lun, priv)  to struct device_subdevice pair. 
    For lun = 0, or unknown (lun, priv) pair, returns (struct device_subdevice){NULL,0} 
 */
 static
 device_subdevice_t cpu__lookup_device_subdevice(const struct lun_priv2device_subdevice_map* dev_subdev_map, 
                                                 lun_t lun, enum privileges priv);

 /* functional description of the delegating xcmd0 .. xcmd7 instructions */
 template<k = 0..N-1> //pretend this is C
 long xcmd<k>(lun_data_t rs1, long rs2)
 {
     struct device_subdevice dev_subdev = cpu__lookup_device_subdevice(device_subdevice_map, rs1.lun, current_privilege());
     if(dev_subdev.devAddr == NULL)
        trap(“Illegal instruction”);

     return dev_subdev.devAddr(dev_subdev.subdevId | k << 12, rs1, rs2);
 }

Example:

 #define com_bigbucks__Frobate__uuid 0xABCDE
 #define org_tinker_tinker__RocknRoll__uuid 0x12345
 #define org_tinker_tinker__Jazz__uuid 0xD0B0D
 /*
 com.bigbucks:Frobate{
     uuid: com_bigbucks__Frobate__uuid
     frobate rd rs1 rs2 : cmd0 rd rs1 rs2
     foo     rd rs1 rs2 : cmd1 rd rs1 rs2
     bar     rd rs1 rs2 : cmd1 rd rs1 rs2
 }
 */
 org.tinker.tinker:RocknRoll{
     uuid: org_tinker_tinker__RocknRoll__uuid
     rock rd rs1 rs2: cmd0 rd rs1 rs2
     roll rd rs1 rs2: cmd1 rd rs1 rs2
 }

 long com_bigbucks__device1(short  subdevice_xcmd, lun_data_t rs1, long rs2)
 {
    switch(subdevice_xcmd) {
    case 0 | 0 << 12  /* com.bigbucks:Frobate:frobate */     : return device1_frobate(rs1, rs2);
    case 42| 0 << 12  /* com.bigbucks:FrobateMach:frobate    : return device1_frobate_machine_level(rs1, rs2);
    case 0 | 1 << 12  /* com.bigbucks:Frobate:foo */         : return device1_foo(rs1, rs2);
    case 0 | 2 << 12  /* com.bigbucks:Frobate:bar */         : return device1_bar(rs1, rs2);
    case 1 | 0 << 12  /* org.tinker.tinker:RocknRoll:rock */ : return device1_rock(rs1, rs2);
    case 1 | 1 << 12  /* org.tinker.tinker:RocknRoll:roll */ : return device1_roll(rs1, rs2);
    default: trap(“hardware configuration error”);
    }
 }

 /*
 org.tinker.tinker:Jazz{
   uuid: org_tinker_tinker__Jazz__uuid 
   boogy rd rs1 rs2: cmd0 rd rs1 rs2
 }
 */

 long org_tinker_tinker__device2(short subdevice_xcmd,  lun_data_t rs1, long rs2)
 {
    switch(dev_cmd.interfId){
    case 0  | 0 << 12 /* com.bigbucks:Frobate:frobate */: return device2_frobate(rs1, rs2);
    case 0  | 1 << 12 /* com.bigbucks:Frobate:foo */    : return device2_foo(rs1, rs2);
    case 0  | 2 << 12 /* com.bigbucks:Frobate:bar */    : return device2_foo(rs1, rs2);
    case 1  | 0 << 12 /* org_tinker_tinker:Jazz:boogy */: return device2_boogy(rs1, rs2);
    default: trap(“hardware configuration error”);      
    }
 }

    /* struct uuid_dev2lun_map[] */  
    lun_map = {     
        {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = user},  .lun =  1},
        {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = super}, .lun =  1},
        {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = hyper}, .lun =  1},
        {{.uuid_devId = {org_RiscV__Fallback__ReturnZero__uuid , 0},    .priv = mach}   .lun =  1},   
        {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = user},  .lun =  2},
        {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = super}, .lun =  2},
        {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = hyper}, .lun =  2},
        {{.uuid_devId = {org_RiscV__Fallback__ReturnMinusOne__uuid, 0}, .priv = mach},  .lun =  2},
        {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = user}   .lun = 32},  //32 sic!
        {{.uuid_devId = {com_bigbucks__Frobate__uuid, 1},               .priv = super}  .lun = 32},
        {{.uuid_devId = {com_bigbucks__Frobate__uuid, 1},               .priv = hyper}  .lun = 32},
        {{.uuid_devId = {com_bigbucks__Frobate__uuid, 1},               .priv = mach}   .lun = 32},
        {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = super}  .lun = 34},  //34 sic!
        {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = hyper}  .lun = 34},  
        {{.uuid_devId = {com_bigbucks__Frobate__uuid, 0},               .priv = mach}   .lun = 34},  
        {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = user}   .lun = 33},  //33 sic!
        {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = super}  .lun = 33},  
        {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = hyper}  .lun = 33},  
        {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = super}, .lun = 35},
        {{.uuid_devId = {org_tinker_tinker__RocknRoll__uuid, 0},        .priv = hyper}, .lun = 35},
   }

 /* struct lun2dev_subdevice_map[] */
    dev_subdevice_map = {
  //     {.lun = 0,   error and falls back to trapping xcmd 
         {{.lun = 1, .priv = user},  .devAddr_interfId = {fallback,    0 /* ReturnZero  */}},
         {{.lun = 1, .priv = super}, .devAddr_interfId = {fallback,    0 /* ReturnZero  */}},
         {{.lun = 1, .priv = hyper}, .devAddr_interfId = {fallback,    0 /* ReturnZero  */}},
         {{.lun = 1, .priv = mach},  .devAddr_interfId = {fallback,    0 /* ReturnZero  */}},
         {{.lun = 2, .priv = user},  .devAddr_interfId = {fallback,    1 /* ReturnMinusOne*/}},
         {{.lun = 2, .priv = super}, .devAddr_interfId = {fallback,    1 /* ReturnMinusOne*/}},
         {{.lun = 2, .priv = hyper}, .devAddr_interfId = {fallback,    1 /* ReturnMinusOne*/}},
         {{.lun = 2, .priv = mach},  .devAddr_interfId = {fallback,    1 /* ReturnMinusOne*/}},
 //       .lun = 3 .. 7  reserved for other fallback RV interfaces
 //       .lun = 8 .. 30 reserved as error numbers, c.li t1 31; bltu rd t1 L_fail tests errors
 //      .lun = 31  reserved out of caution 
         {{.lun = 32, .priv = user},  .devAddr_interfId = {device1, 0 /* Frobate  interface */}},
         {{.lun = 32, .priv = super}, .devAddr_interfId = {device1, 0 /* Frobate  interface */}},
         {{.lun = 32, .priv = hyper}, .devAddr_interfId = {device1, 0 /* Frobate  interface */}},
         {{.lun = 32, .priv = mach},  .devAddr_interfId = {device1,64 /* Frobate  machine level interface */}},
         {{.lun = 33, .priv = user},  .devAddr_InterfId = {device1, 1 /* RocknRoll interface */}},
         {{.lun = 33, .priv = super}, .devAddr_InterfId = {device1, 1 /* RocknRoll interface */}},
         {{.lun = 33, .priv = hyper}, .devAddr_InterfId = {device1, 1 /* RocknRoll interface */}},
         {{.lun = 34, .priv = super}, .devAddr_interfId = {device2, 0 /* Frobate interface */}},
         {{.lun = 34, .priv = hyper}, .devAddr_interfId = {device2, 0 /* Frobate interface */}},
         {{.lun = 34, .priv = mach},  .devAddr_interfId = {device2, 0 /* Frobate interface */}},
         {{.lun = 35, .priv = super}, .devAddr_interfId = {device2, 1 /* Jazz interface */}},
         {{.lun = 35, .priv = hyper}, .devAddr_interfId = {device2, 1 /* Jazz interface */}},
     }