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[rtl] Fix issue with ID/EX exceptions

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When a potential exception occurs in ID/EX controller must wait for any
outstanding instruction in WB to complete before resolving it. The
instruction in WB may also have an exception which takes priority over
ID/EX.

Signed-off-by: Greg Chadwick <gac@lowrisc.org>
This commit is contained in:
Greg Chadwick 2020-06-01 16:29:40 +01:00
parent 833bdefe33
commit c0ae8ffb99
2 changed files with 13 additions and 2 deletions
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14
rtl/ibex_controller.sv
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@ -94,6 +94,7 @@ module ibex_controller #(
input logic stall_id_i,
input logic stall_wb_i,
output logic flush_id_o,
input logic ready_wb_i,
// performance monitors
output logic perf_jump_o, // we are executing a jump
@ -482,8 +483,17 @@ module ibex_controller #(
// Halt IF but don't flush ID. This leaves a valid instruction in
// ID so controller can determine appropriate action in the
// FLUSH state.
ctrl_fsm_ns = FLUSH;
halt_if = 1'b1;
halt_if = 1'b1;
// Wait for the writeback stage to either be ready for a new instruction or raise its own
// exception before going to FLUSH. If the instruction in writeback raises an exception it
// must take priority over any exception from an instruction in ID/EX. Only once the
// writeback stage is ready can we be certain that won't happen. Without a writeback
// stage ready_wb_i == 1 so the FSM will always go directly to FLUSH.
if (ready_wb_i | wb_exception_o) begin
ctrl_fsm_ns = FLUSH;
end
end
if ((branch_set_i || jump_set_i) && !special_req_branch) begin

1
rtl/ibex_id_stage.sv
View file

@ -590,6 +590,7 @@ module ibex_id_stage #(
.stall_id_i ( stall_id ),
.stall_wb_i ( stall_wb ),
.flush_id_o ( flush_id ),
.ready_wb_i ( ready_wb_i ),
// Performance Counters
.perf_jump_o ( perf_jump_o ),