execute1: Simplify the interrupt logic a little

This makes some simplifications to the interrupt logic which will
help with later commits.

- When irq_valid is set, don't set exception to 1 until we have a
  valid instruction.  That means we can remove the if e_in.valid = '1'
  test from the exception = '1' block.

- Don't assert stall_out on the first cycle of delivering an
  interrupt.  If we do get another instruction in the next cycle,
  nothing will happen because we have ctrl.irq_state set and we
  will just continue writing the interrupt registers.

- Make sure we deliver as many completions as we got instructions,
  otherwise the outstanding instruction count in control.vhdl gets
  out of sync.

- In writeback, make sure all of the other write enables are ignored
  when e_in.exc_write_enable is set.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

execute1.vhdl

@@ -399,13 +399,15 @@ begin
 	    ctrl_tmp.msr(63 - 48) <= '0'; -- clear EE
 	    f_out.redirect <= '1';
 	    f_out.redirect_nia <= ctrl.irq_nia;
-	    v.e.valid := '1';
+	    v.e.valid := e_in.valid;
 	    report "Writing SRR1: " & to_hstring(ctrl.srr1);
 
 	elsif irq_valid = '1' then
 	    -- we need two cycles to write srr0 and 1
 	    -- will need more when we have to write DSISR, DAR and HIER
-	    exception := '1';
+            -- Don't deliver the interrupt until we have a valid instruction
+            -- coming in, so we have a valid NIA to put in SRR0.
+	    exception := e_in.valid;
 	    ctrl_tmp.irq_nia <= std_logic_vector(to_unsigned(16#900#, 64));
 	    ctrl_tmp.srr1 <= msr_copy(ctrl.msr);
 
@@ -821,16 +823,12 @@ begin
 	end if;
 
 	if exception = '1' then
-	    if e_in.valid = '1' then
             v.e.exc_write_enable := '1';
             if exception_nextpc = '1' then
-                    v.e.exc_write_data := std_logic_vector(unsigned(e_in.nia) + 4);
+                v.e.exc_write_data := next_nia;
             end if;
             ctrl_tmp.irq_state <= WRITE_SRR1;
-		stall_out <= '1';
+            v.e.valid := '1';
-		v.e.valid := '0';
-                result_en := '0';
-	    end if;
 	end if;
 
 	v.e.write_data := result;

writeback.vhdl

@@ -55,7 +55,8 @@ begin
             w_out.write_reg <= e_in.exc_write_reg;
             w_out.write_data <= e_in.exc_write_data;
             w_out.write_enable <= '1';
-        elsif e_in.write_enable = '1' then
+        else
+            if e_in.write_enable = '1' then
                 w_out.write_reg <= e_in.write_reg;
                 w_out.write_data <= e_in.write_data;
                 w_out.write_enable <= '1';
@@ -102,5 +103,6 @@ begin
                 cf(0) := e_in.xerc.so;
                 c_out.write_cr_data(31 downto 28) <= cf;
             end if;
+        end if;
     end process;
 end;