@ -1,3 +1,21 @@
--
-- Set associative icache
--
-- TODO (in no specific order):
--
-- * Add debug interface to inspect cache content
-- * Add snoop/invalidate path
-- * Add multi-hit error detection
-- * Pipelined bus interface (wb or axi)
-- * Maybe add parity ? There's a few bits free in each BRAM row on Xilinx
-- * Add optimization: service hits on partially loaded lines
-- * Add optimization: (maybe) interrupt reload on fluch/redirect
-- * Check if playing with the geometry of the cache tags allow for more
-- efficient use of distributed RAM and less logic/muxes. Currently we
-- write TAG_BITS width which may not match full ram blocks and might
-- cause muxes to be inferred for "partial writes".
-- * Check if making the read size of PLRU a ROM helps utilization
--
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
@ -11,9 +29,11 @@ use work.wishbone_types.all;
entity icache is
generic (
-- Line size in bytes
LINE_SIZE : natural := 64;
-- Number of lines
NUM_LINES : natural := 32
LINE_SIZE : positive := 64;
-- Number of lines in a set
NUM_LINES : positive := 32;
-- Number of ways
NUM_WAYS : positive := 4
);
port (
clk : in std_ulogic;
@ -80,6 +100,8 @@ architecture rtl of icache is
constant INDEX_BITS : natural := log2(NUM_LINES);
-- TAG_BITS is the number of bits of the tag part of the address
constant TAG_BITS : natural := 64 - LINE_OFF_BITS - INDEX_BITS;
-- WAY_BITS is the number of bits to select a way
constant WAY_BITS : natural := log2(NUM_WAYS);
-- Example of layout for 32 lines of 64 bytes:
--
@ -96,30 +118,38 @@ architecture rtl of icache is
subtype row_t is integer range 0 to BRAM_ROWS-1;
subtype index_t is integer range 0 to NUM_LINES-1;
subtype way_t is integer range 0 to NUM_WAYS-1;
-- The cache data BRAM organized as described above
subtype cache_row_t is std_logic_vector(wishbone_data_bits-1 downto 0);
type cache_array is array(row_t) of cache_row_t;
-- The cache data BRAM organized as described above for each way
subtype cache_row_t is std_ulogic_vector(wishbone_data_bits-1 downto 0);
-- The cache tags LUTRAM has a row per cache line
-- The cache tags LUTRAM has a row per set. Vivado is a pain and will
-- not handle a clean (commented) definition of the cache tags as a 3d
-- memory. For now, work around it by putting all the tags
subtype cache_tag_t is std_logic_vector(TAG_BITS-1 downto 0);
type cache_tags_array is array(index_t) of cache_tag_t;
-- type cache_tags_set_t is array(way_t) of cache_tag_t;
-- type cache_tags_array_t is array(index_t) of cache_tags_set_t;
constant TAG_RAM_WIDTH : natural := TAG_BITS * NUM_WAYS;
subtype cache_tags_set_t is std_logic_vector(TAG_RAM_WIDTH-1 downto 0);
type cache_tags_array_t is array(index_t) of cache_tags_set_t;
-- The cache valid bits
subtype cache_way_valids_t is std_ulogic_vector(NUM_WAYS-1 downto 0);
type cache_valids_t is array(index_t) of cache_way_valids_t;
-- Storage. Hopefully "cache_rows" is a BRAM, the rest is LUTs
signal cache_rows : cache_array;
signal tags : cache_tags_array;
signal tags_valid : std_ulogic_vector(NUM_LINES-1 downto 0);
signal cache_tags : cache_tags_array_t;
signal cache_valids : cache_valids_t;
attribute ram_style : string;
attribute ram_style of cache_rows : signal is "block";
attribute ram_decomp : string;
attribute ram_decomp of cache_rows : signal is "power";
attribute ram_style of cache_tags : signal is "distributed";
-- Cache reload state machine
type state_t is (IDLE, WAIT_ACK);
type reg_internal_t is record
-- Cache hit state (1 cycle BRAM access)
hit_row : cache_row_t;
-- Cache hit state (Latches for 1 cycle BRAM access)
hit_way : way_t;
hit_nia : std_ulogic_vector(63 downto 0);
hit_smark : std_ulogic;
hit_valid : std_ulogic;
@ -127,16 +157,27 @@ architecture rtl of icache is
-- Cache miss state (reload state machine)
state : state_t;
wb : wishbone_master_out;
store_way : way_t;
store_index : index_t;
end record;
signal r : reg_internal_t;
-- Async signals on incoming request
signal req_index : index_t;
signal req_row : row_t;
signal req_tag : cache_tag_t;
signal req_is_hit : std_ulogic;
signal req_index : index_t;
signal req_row : row_t;
signal req_hit_way : way_t;
signal req_tag : cache_tag_t;
signal req_is_hit : std_ulogic;
signal req_is_miss : std_ulogic;
-- Cache RAM interface
type cache_ram_out_t is array(way_t) of cache_row_t;
signal cache_out : cache_ram_out_t;
-- PLRU output interface
type plru_out_t is array(index_t) of std_ulogic_vector(WAY_BITS-1 downto 0);
signal plru_victim : plru_out_t;
-- Return the cache line index (tag index) for an address
function get_index(addr: std_ulogic_vector(63 downto 0)) return index_t is
@ -185,7 +226,22 @@ architecture rtl of icache is
return addr(63 downto 64-TAG_BITS);
end;
-- Read a tag from a tag memory row
function read_tag(way: way_t; tagset: cache_tags_set_t) return cache_tag_t is
begin
return tagset((way+1) * TAG_BITS - 1 downto way * TAG_BITS);
end;
-- Write a tag to tag memory row
procedure write_tag(way: in way_t; tagset: inout cache_tags_set_t;
tag: cache_tag_t) is
begin
tagset((way+1) * TAG_BITS - 1 downto way * TAG_BITS) := tag;
end;
begin
assert LINE_SIZE mod ROW_SIZE = 0;
assert ispow2(LINE_SIZE) report "LINE_SIZE not power of 2" severity FAILURE;
assert ispow2(NUM_LINES) report "NUM_LINES not power of 2" severity FAILURE;
assert ispow2(ROW_PER_LINE) report "ROW_PER_LINE not power of 2" severity FAILURE;
@ -212,66 +268,134 @@ begin
report "ROW_OFF_BITS = " & natural'image(ROW_OFF_BITS);
report "INDEX_BITS = " & natural'image(INDEX_BITS);
report "TAG_BITS = " & natural'image(TAG_BITS);
report "WAY_BITS = " & natural'image(WAY_BITS);
wait;
end process;
-- Generate a cache RAM for each way
rams: for i in 0 to NUM_WAYS-1 generate
signal do_write : std_ulogic;
signal rd_addr : std_ulogic_vector(ROW_BITS-1 downto 0);
signal wr_addr : std_ulogic_vector(ROW_BITS-1 downto 0);
signal dout : cache_row_t;
begin
way: entity work.cache_ram
generic map (
ROW_BITS => ROW_BITS,
WIDTH => wishbone_data_bits
)
port map (
clk => clk,
rd_en => '1', -- fixme
rd_addr => rd_addr,
rd_data => dout,
wr_en => do_write,
wr_addr => wr_addr,
wr_data => wishbone_in.dat
);
process(all)
begin
do_write <= '0';
if wishbone_in.ack = '1' and r.store_way = i then
do_write <= '1';
end if;
cache_out(i) <= dout;
rd_addr <= std_ulogic_vector(to_unsigned(req_row, ROW_BITS));
wr_addr <= std_ulogic_vector(to_unsigned(get_row(r.wb.adr), ROW_BITS));
end process;
end generate;
-- Generate PLRUs
maybe_plrus: if NUM_WAYS > 1 generate
begin
plrus: for i in 0 to NUM_LINES-1 generate
-- PLRU interface
signal plru_acc : std_ulogic_vector(WAY_BITS-1 downto 0);
signal plru_acc_en : std_ulogic;
signal plru_out : std_ulogic_vector(WAY_BITS-1 downto 0);
begin
plru : entity work.plru
generic map (
BITS => WAY_BITS
)
port map (
clk => clk,
rst => rst,
acc => plru_acc,
acc_en => plru_acc_en,
lru => plru_out
);
process(req_index, req_is_hit, req_hit_way, req_is_hit, plru_out)
begin
-- PLRU interface
if req_is_hit = '1' and req_index = i then
plru_acc_en <= req_is_hit;
else
plru_acc_en <= '0';
end if;
plru_acc <= std_ulogic_vector(to_unsigned(req_hit_way, WAY_BITS));
plru_victim(i) <= plru_out;
end process;
end generate;
end generate;
-- Cache hit detection, output to fetch2 and other misc logic
icache_comb : process(all)
variable is_hit : std_ulogic;
variable hit_way : way_t;
begin
-- Extract line, row and tag from request
req_index <= get_index(i_in.nia);
req_row <= get_row(i_in.nia);
req_tag <= get_tag(i_in.nia);
-- Test if pending request is a hit
if tags(req_index) = req_tag then
req_is_hit <= tags_valid(req_index);
else
req_is_hit <= '0';
end if;
-- Test if pending request is a hit on any way
hit_way := 0;
is_hit := '0';
for i in way_t loop
if read_tag(i, cache_tags(req_index)) = req_tag and
cache_valids(req_index)(i) = '1' then
hit_way := i;
is_hit := '1';
end if;
end loop;
-- Generate the "hit" and "miss" signals for the synchronous blocks
req_is_hit <= i_in.req and is_hit and not flush_in;
req_is_miss <= i_in.req and not is_hit and not flush_in;
req_hit_way <= hit_way;
-- Output instruction from current cache row
--
-- Note: This is a mild violation of our design principle of having pipeline
-- stages output from a clean latch. In this case we output the result
-- of a mux. The alternative would be output an entire cache line
-- which I prefer not to do just yet.
-- of a mux. The alternative would be output an entire row which
-- I prefer not to do just yet as it would force fetch2 to know about
-- some of the cache geometry information.
--
i_out.insn <= read_insn_word(r.hit_nia, r.hit_row);
i_out.insn <= read_insn_word(r.hit_nia, cache_out(r.hit_way));
i_out.valid <= r.hit_valid;
i_out.nia <= r.hit_nia;
i_out.stop_mark <= r.hit_smark;
-- This needs to match the latching of a new request in process icache_hit
stall_out <= not req_is_hit;
-- Stall fetch1 if we have a miss
stall_out <= not is_hit;
-- Wishbone requests output (from the cache miss reload machine)
wishbone_out <= r.wb;
end process;
-- Cache hit synchronous machine
icache_hit : process(clk)
begin
if rising_edge(clk) then
-- Debug
if i_in.req = '1' then
report "cache search for " & to_hstring(i_in.nia) &
" index:" & integer'image(req_index) &
" row:" & integer'image(req_row) &
" want_tag:" & to_hstring(req_tag) & " got_tag:" & to_hstring(req_tag) &
" valid:" & std_ulogic'image(tags_valid(req_index));
if req_is_hit = '1' then
report "is hit !";
else
report "is miss !";
end if;
end if;
-- Are we free to latch a new request ?
--
-- Note: this test needs to match the equation for generating stall_out
-- On a hit, latch the request for the next cycle, when the BRAM data
-- will be available on the cache_out output of the corresponding way
--
if i_in.req = '1' and req_is_hit = '1' and flush_in = '0' then
-- Read the cache line (BRAM read port) and remember the NIA
r.hit_row <= cache_rows(req_row);
if req_is_hit = '1' then
r.hit_way <= req_hit_way;
r.hit_nia <= i_in.nia;
r.hit_smark <= i_in.stop_mark;
r.hit_valid <= '1';
@ -279,20 +403,28 @@ begin
report "cache hit nia:" & to_hstring(i_in.nia) &
" SM:" & std_ulogic'image(i_in.stop_mark) &
" idx:" & integer'image(req_index) &
" tag:" & to_hstring(req_tag);
" tag:" & to_hstring(req_tag) &
" way: " & integer'image(req_hit_way);
else
r.hit_valid <= '0';
-- Send stop marks down regardless of validity
r.hit_smark <= i_in.stop_mark;
end if;
end if;
end process;
-- Cache miss/reload synchronous machine
icache_miss : process(clk)
variable way : integer range 0 to NUM_WAYS-1;
variable tagset : cache_tags_set_t;
begin
if rising_edge(clk) then
-- On reset, clear all valid bits to force misses
if rst = '1' then
tags_valid <= (others => '0');
for i in index_t loop
cache_valids(i) <= (others => '0');
end loop;
r.state <= IDLE;
r.wb.cyc <= '0';
r.wb.stb <= '0';
@ -302,23 +434,38 @@ begin
r.wb.sel <= "11111111";
r.wb.we <= '0';
else
-- State machine
-- Main state machine
case r.state is
when IDLE =>
-- We need to read a cache line
if i_in.req = '1' and req_is_hit = '0' then
if req_is_miss = '1' then
way := to_integer(unsigned(plru_victim(req_index)));
report "cache miss nia:" & to_hstring(i_in.nia) &
" SM:" & std_ulogic'image(i_in.stop_mark) &
" idx:" & integer'image(req_index) &
" way:" & integer'image(way) &
" tag:" & to_hstring(req_tag);
-- Force misses while reloading that line
tags_valid(req_index) <= '0';
tags(req_index) <= req_tag;
-- Force misses on that way while reloading that line
cache_valids(req_index)(way) <= '0';
-- Store new tag in selected way
for i in 0 to NUM_WAYS-1 loop
if i = way then
tagset := cache_tags(req_index);
write_tag(i, tagset, req_tag);
cache_tags(req_index) <= tagset;
end if;
end loop;
-- Keep track of our index and way for subsequent stores
r.store_index <= req_index;
r.store_way <= way;
-- Prep for first wishbone read. We calculate the address off
-- Prep for first wishbone read. We calculate the address of
-- the start of the cache line
--
r.wb.adr <= i_in.nia(63 downto LINE_OFF_BITS) &
(LINE_OFF_BITS-1 downto 0 => '0');
r.wb.cyc <= '1';
@ -328,12 +475,9 @@ begin
end if;
when WAIT_ACK =>
if wishbone_in.ack = '1' then
-- Store the current dword in both the cache
cache_rows(get_row(r.wb.adr)) <= wishbone_in.dat;
-- That was the last word ? We are done
if is_last_row(r.wb.adr) then
tags_valid(r.store_index) <= '1';
cache_valids(r.store_index)(way) <= '1';
r.wb.cyc <= '0';
r.wb.stb <= '0';
r.state <= IDLE;
Benjamin Herrenschmidt
5 years ago