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Fix priority inversion for two back-to-back divides (#948)

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If the first one is killed for some unrelated reason (e.g. write port
hazard), the second one will still issue to the div-sqrt unit.  While
it will itself later be killed, the fact that the later instruction
acquires a resource needed by the former instruction leads to deadlock.
This commit is contained in:
Andrew Waterman
2017-08-10 17:12:09 -07:00
committed by GitHub
parent fa867bc478
commit a3358f34a0
1 changed files with 9 additions and 4 deletions
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13
src/main/scala/tile/FPU.scala
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@@ -581,12 +581,17 @@ class FPU(cfg: FPUParams)(implicit p: Parameters) extends FPUModule()(p) {
val req_valid = ex_reg_valid || io.cp_req.valid
val ex_reg_inst = RegEnable(io.inst, io.valid)
val ex_cp_valid = io.cp_req.fire()
val mem_reg_valid = Reg(next=ex_reg_valid && !io.killx || ex_cp_valid, init=Bool(false))
val mem_reg_inst = RegEnable(ex_reg_inst, ex_reg_valid)
val mem_cp_valid = Reg(next=ex_cp_valid, init=Bool(false))
val killm = (io.killm || io.nack_mem) && !mem_cp_valid
val wb_reg_valid = Reg(next=mem_reg_valid && (!killm || mem_cp_valid), init=Bool(false))
val wb_cp_valid = Reg(next=mem_cp_valid, init=Bool(false))
val mem_reg_valid = RegInit(false.B)
val killm = (io.killm || io.nack_mem) && !mem_cp_valid
// Kill X-stage instruction if M-stage is killed. This prevents it from
// speculatively being sent to the div-sqrt unit, which can cause priority
// inversion for two back-to-back divides, the first of which is killed.
val killx = io.killx || mem_reg_valid && killm
mem_reg_valid := ex_reg_valid && !killx || ex_cp_valid
val mem_reg_inst = RegEnable(ex_reg_inst, ex_reg_valid)
val wb_reg_valid = Reg(next=mem_reg_valid && (!killm || mem_cp_valid), init=Bool(false))
val fp_decoder = Module(new FPUDecoder)
fp_decoder.io.inst := io.inst