core: Implement reserved/no-op SPR numbers

SPR numbers 808 - 811 do nothing when read or written, that is, mfspr
doesn't modify the destination register.  This is accomplished in the
same way that privileged mfspr to an unimplemented SPR is made a
no-op, by supplying the old contents of the destination register as an
input and writing that same value back.

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

common.vhdl

@@ -75,6 +75,10 @@ package common is
     constant SPR_DEXCRU : spr_num_t := 812;
     constant SPR_HDEXCR : spr_num_t := 471;
     constant SPR_HDEXCU : spr_num_t := 455;
+    constant SPR_NOOP0  : spr_num_t := 808;
+    constant SPR_NOOP1  : spr_num_t := 809;
+    constant SPR_NOOP2  : spr_num_t := 810;
+    constant SPR_NOOP3  : spr_num_t := 811;
 
     -- PMU registers
     constant SPR_UPMC1  : spr_num_t := 771;
@@ -171,6 +175,7 @@ package common is
         ispmu : std_ulogic;
         ronly : std_ulogic;
         wonly : std_ulogic;
+        noop  : std_ulogic;
     end record;
     constant spr_id_init : spr_id := (sel => "0000", others => '0');
 

decode1.vhdl

@@ -457,6 +457,7 @@ architecture behaviour of decode1 is
         i.ispmu := '0';
         i.ronly := '0';
         i.wonly := '0';
+        i.noop  := '0';
         case sprn is
             when SPR_TB =>
                 i.sel := SPRSEL_TB;
@@ -504,6 +505,8 @@ architecture behaviour of decode1 is
             when SPR_DEXCRU | SPR_HDEXCU =>
                 i.sel := SPRSEL_DEXCR;
                 i.ronly := '1';
+            when SPR_NOOP0 | SPR_NOOP1 | SPR_NOOP2 | SPR_NOOP3 =>
+                i.noop := '1';
             when others =>
                 i.valid := '0';
         end case;

decode2.vhdl

@@ -696,9 +696,11 @@ begin
             if op = OP_MFSPR then
                 if d_in.ram_spr.valid = '1' then
                     v.e.result_sel := "101";        -- ramspr_result
-                elsif d_in.spr_info.valid = '0' or d_in.spr_info.wonly = '1' then
+                elsif d_in.spr_info.valid = '0' or d_in.spr_info.wonly = '1' or
+                    d_in.spr_info.noop = '1' then
                     -- Privileged mfspr to invalid/unimplemented SPR numbers
                     -- writes the contents of RT back to RT (i.e. it's a no-op)
+                    -- as does any mfspr from the reserved/noop SPR numbers
                     v.e.result_sel := "001";        -- logical_result
                 end if;
             end if;

execute1.vhdl

@@ -1332,7 +1332,7 @@ begin
             when OP_DARN =>
 	    when OP_MFMSR =>
 	    when OP_MFSPR =>
-		if e_in.spr_is_ram = '1' then
+		if e_in.spr_is_ram = '1' or e_in.spr_select.noop = '1' then
                     if e_in.valid = '1' and not is_X(e_in.insn) then
                         report "MFSPR to SPR " & integer'image(decode_spr_num(e_in.insn)) &
                             "=" & to_hstring(alu_result);