The Remote RoCC (ReRoCC) extensions for RoCC-enabled RISC-V cores enables physical disaggregation of RoCC accelerators from control cores. To simplify adoption and exploration, ReRoCC is backwards compatible with existing RoCC accelerator implementations and software stacks. ReRoCC provides a path towards supporting "accelerator virtualization" on many-accelerator architectures, where threads can contend for and share limited physical accelerator resources without requiring user-level software modifications.
The accelerator client attaches to existing RISC-V cores to enable them to use remote ReRoCC accelerators. The client is itself a RoCC "accelerator", and can plug-in to existing cores like Rocket, (TODO) BOOM, and (TODO) Shuttle.
The client is responsible for tracking which physical accelerators are acquired by the attached hart, forwarding accelerator instructions, and maintaining shadowed architectural state on the remote accelerators.
The accelerator manager wraps an existing RoCC accelerator implementation, providing the standard RoCC interface to the accelerator.
The manager implements shadow copies of critical core architectural state, including the mstatus and ptbr registers, as well as a shadow page-table-walker, for the accelerator to access.
ReRoCC adds new user-level CSRs to the host CPU, which manage the availability of physical accelerators for the CPU thread.
The rrcfg registers track an "acquired" accelerator available for the hart to access.
Implementations may support up to 16 rrcfg registers.
Each rrcfg register contains a mgr field, which can be set to the physical ID of a remote RoCC accelerator manager, and a acq field, which, if set, indicates that the accelerator is available to access.
The rropc registers assign rrcfg registers to the four available RoCC custom opcodes.
When a rropcX register is set to the index of a rrcfg register with the acq bit set, then instructions with opcode customX will be send to that accelerator.
The rrbar register is currently used to assist in fencing accelerator memory accesses. This will be removed in the future.
| CSR Id | Name |
|---|---|
| 0x800-0x803 | rropc0-rropc3 |
| 0x804 | rrbar |
| 0x810-0x81f | rrcfg0-rrcfg15 |
| ReRoCC CSR Field | Description |
|---|---|
rropcX[3:0] | Index of rrcfg opcode customX is mapped to |
|
rrbar[3:0] | When written, indicates that the next memory fence should apply to accelerator managed by the indexed rrcfg |
|
rrcfgX[7:0] |
Index of physical accelerator this register is mapped to |
rrcfgX[8] |
If set, indicates this register should attempt to "acquire" the corresponding physical accelerator. Reading this bit returns whether acquisition was successful |
The ReRoCC messaging protocol is a simple, in-order, non-blocking, dual-channel packetized messaging protocol to facilitate communication and synchronization between accelerator clients and managers.
A request channel is for client-to-manager traffic, while a response channel implements manager-to-client traffic.
Both channels use the same message encoding.
The protocol can be implemented by a crossbar or NoC-based interconnect. The protocol implements a 64-bit data channel with multi-beat message types.
| Field | Width | Description |
|---|---|---|
| opcode | 3 | Type of the message. See the opcode table. |
| client_id | ? | ID of the rrcfg register on one end of the message. Managers see unique IDs per rrcfg on a multi-hart system |
| manager_id | ? | ID of the physical manager on one end of the message. |
| data | 64 | See opcode table |
Opcode |
Value | Total Beats | Beat | data[63:0] |
|---|---|---|---|---|
| mAcquire | 0b000 |
1 | 0 | Ignored |
| mInst | 0b001 |
1-3 | 0 | 32-bit custom instruction |
| 1 | (If requested) rs1 operand data | |||
| 2 | (If requested) rs2 operand data | |||
| mUStatus | 0b010 | 2 | 0 | New csr_mstatus[63:0] |
|||
| 1 | New csr_mstatus[127:64] |
|||
| mUPtbr | 0b011 | 1 | 0 | New csr_ptbr |
|||
| mRelease | 0b100 |
1 | 0 | Ignored |
| mUnbusy | 0b101 |
1 | 0 | Ignored |
Opcode |
Value | Total Beats | Beat | data[63:0] |
|---|---|---|---|---|
| sAcqResp | 0b000 | 1 | 0 | 1 if successful acquire, else 0 |
|||
| sInstAck | 0b001 |
1 | 0 | Ignored |
| sWrite | 0b010 |
2 | 0 | 64-bit register write data |
| 1 | 64-bit architectural register destination | |||
| sRelResp | 0b011 |
1 | 0 | Ignored |
| sUnbusyAck | 0b100 |
1 | 0 | Ignored |
| Request Message | Response Message | Description |
|---|---|---|
| mAcquire | mAcqResp | Attempt to acquire a ReRoCC-managed accelerator, manager responds whether acquire was successful |
| mInst | mInstAck | Sends an instruction stream to the manager, credit-based flow-control with Inst/InstAck |
| mUStatus | Updates shadowed MStatus on the manager | |
| mUPtbr | Updates shadowed satp on the manager | |
| mRelease | mRelResp | Relinquishes an attached accelerator |
| mUnbusy | mUnbusyAck | Wait for accelerator to unbusy (TODO: Remove) |
ReRoCC is currently under development. A brief list of TODOs:
rrbar csr.