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icache: Read icache tag RAM synchronously

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This uses the next_nia provided to us by fetch1 to enable the icache
tag RAM to be read synchronously (using a clock edge), which should
enable block RAMs to be used on FPGAs rather than LUT RAM or
flip-flops.  We define a separate RAM per way to avoid any problems
with the tools trying to inference byte write enables for writing to a
single way.

Since next_nia can move on, we only get one shot at reading it the
cache tag RAM entry for the current access.  If it is a miss, then the
state machine will read the cache line from RAM, and we can consider
the access to be a hit once the state machine has brought in the
doubleword we need.  The TLB hit/miss check has been modified to check
r.store_tag rather than the tag read from the tag RAM for this case.

However, it is also possible that stall_in will be asserted for the
whole time until the cache line refill is completed.  To handle this
case, we remember (in r.stalled_hit) that we detected a hit while
stalled, and use that hit once stall_in is deasserted.  This avoids
doing an unnecesary second reload of the same cache line.  The
r.stalled_hit flag gets cleared in CLR_TAG state since that is when
cache tags can be overwritten, meaning that a previously detected hit
might no longer be valid.

There is also the case where the tag read from the tag RAM is the one
we are looking for, and is the same index as the line that is starting
to be reloaded by the state machine.  If the icache gets stalled for
long enough that the line reload finishes, it would then be possible
for the access to be detected as a hit even though the cache line has
been overwritten.  To counter this, we detect the case where the cache
tag RAM entry being read is the same as the entry being written and
set a 'tag_overwrite' flag bit to indicate that one of the tags in
cache_tags_set is no longer valid.

For snooping writes to memory, we have a second read port on the cache
tag RAM.  These tags are also read synchronously, so the logic for
clearing cache line valid bits on a snoop has been adjusted (the tag
comparisons and valid bit clearing now happen in the same cycle).

This also simplifies the expression for 'insn' by removing a
dependency on r.hit_valid, fixes the instruction value sent to the
log, and deasserts stall_out when flush_in is true.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
pull/422/head
Paul Mackerras 1 year ago
parent 723008b8c2
commit 963c225955
1 changed files with 114 additions and 75 deletions
Show Stats Download Patch File Download Diff File

189
icache.vhdl
Unescape Escape View File

@ -139,28 +139,24 @@ architecture rtl of icache is
-- The cache data BRAM organized as described above for each way
subtype cache_row_t is std_ulogic_vector(ROW_WIDTH-1 downto 0);

-- 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
-- We define a cache tag RAM per way, accessed synchronously
subtype cache_tag_t is std_logic_vector(TAG_BITS-1 downto 0);
-- 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;
type cache_tags_set_t is array(way_t) of cache_tag_t;
type cache_tags_array_t is array(index_t) of cache_tag_t;

-- Set of cache tags read on the last clock edge
signal cache_tags_set : cache_tags_set_t;
-- Set of cache tags for snooping writes to memory
signal snoop_tags_set : cache_tags_set_t;
-- Flags indicating write-hit-read on the cache tags
signal tag_overwrite : std_ulogic_vector(NUM_WAYS - 1 downto 0);

-- 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;
type row_per_line_valid_t is array(0 to ROW_PER_LINE - 1) of std_ulogic;

-- Storage. Hopefully "cache_rows" is a BRAM, the rest is LUTs
signal cache_tags : cache_tags_array_t;
signal cache_valids : cache_valids_t;

attribute ram_style : string;
attribute ram_style of cache_tags : signal is "distributed";

-- L1 ITLB.
constant TLB_BITS : natural := log2(TLB_SIZE);
constant TLB_EA_TAG_BITS : natural := 64 - (TLB_LG_PGSZ + TLB_BITS);
@ -216,6 +212,9 @@ architecture rtl of icache is
end_row_ix : row_in_line_t;
rows_valid : row_per_line_valid_t;

stalled_hit : std_ulogic; -- remembers hit while stalled
stalled_way : way_sig_t;

-- TLB miss state
fetch_failed : std_ulogic;
end record;
@ -248,9 +247,11 @@ architecture rtl of icache is
signal plru_victim : way_sig_t;

-- Memory write snoop signals
signal snoop_valid : std_ulogic;
signal snoop_index : index_sig_t;
signal snoop_hits : cache_way_valids_t;
signal snoop_valid : std_ulogic;
signal snoop_index : index_sig_t;
signal snoop_tag : cache_tag_t;
signal snoop_index2 : index_sig_t;
signal snoop_hits : cache_way_valids_t;

signal log_insn : std_ulogic_vector(35 downto 0);

@ -329,19 +330,6 @@ architecture rtl of icache is
return endian & addr(addr'left downto SET_SIZE_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;

-- Simple hash for direct-mapped TLB index
function hash_ea(addr: std_ulogic_vector(63 downto 0)) return std_ulogic_vector is
variable hash : std_ulogic_vector(TLB_BITS - 1 downto 0);
@ -423,7 +411,9 @@ begin
signal wr_addr : std_ulogic_vector(ROW_BITS-1 downto 0);
signal dout : cache_row_t;
signal wr_sel : std_ulogic_vector(0 downto 0);
signal ic_tags : cache_tags_array_t;
begin
-- Cache data RAMs, one per way
way: entity work.cache_ram
generic map (
ROW_BITS => ROW_BITS,
@ -451,6 +441,47 @@ begin
wr_addr <= std_ulogic_vector(r.store_row);
wr_sel(0) <= do_write;
end process;

-- Cache tag RAMs, one per way, are read and written synchronously.
-- They are instantiated like this instead of trying to describe them as
-- a single array in order to avoid problems with writing a single way.
process(clk)
variable replace_way : way_sig_t;
variable snoop_addr : real_addr_t;
begin
replace_way := to_unsigned(0, WAY_BITS);
if NUM_WAYS > 1 then
-- Get victim way from plru
replace_way := plru_victim;
end if;
if rising_edge(clk) then
-- Read tags using NIA for next cycle
if flush_in = '1' or i_in.req = '0' or (stall_in = '0' and stall_out = '0') then
cache_tags_set(i) <= ic_tags(to_integer(get_index(i_in.next_nia)));
-- Check for simultaneous write to the same location
tag_overwrite(i) <= '0';
if r.state = CLR_TAG and r.store_index = get_index(i_in.next_nia) and
to_unsigned(i, WAY_BITS) = replace_way then
tag_overwrite(i) <= '1';
end if;
end if;

-- Second read port for snooping writes to memory
if (wb_snoop_in.cyc and wb_snoop_in.stb and wb_snoop_in.we) = '1' then
snoop_addr := addr_to_real(wb_to_addr(wb_snoop_in.adr));
snoop_tags_set(i) <= ic_tags(to_integer(get_index(snoop_addr)));
end if;

-- Write one tag when in CLR_TAG state
if r.state = CLR_TAG and to_unsigned(i, WAY_BITS) = replace_way then
ic_tags(to_integer(r.store_index)) <= r.store_tag;
end if;

if rst = '1' then
tag_overwrite(i) <= '0';
end if;
end if;
end process;
end generate;
-- Generate PLRUs
@ -616,17 +647,24 @@ begin
end if;
for i in way_t loop
if i_in.req = '1' and
(cache_valids(to_integer(req_index))(i) = '1' or
(r.state = WAIT_ACK and
req_index = r.store_index and
to_unsigned(i, WAY_BITS) = r.store_way and
r.rows_valid(to_integer(req_row(ROW_LINEBITS-1 downto 0))) = '1')) then
if read_tag(i, cache_tags(to_integer(req_index))) = req_tag then
hit_way := to_unsigned(i, WAY_BITS);
is_hit := '1';
end if;
cache_valids(to_integer(req_index))(i) = '1' and
tag_overwrite(i) = '0' and
cache_tags_set(i) = req_tag then
hit_way := to_unsigned(i, WAY_BITS);
is_hit := '1';
end if;
end loop;
if r.state = WAIT_ACK and r.store_valid = '1' and
req_index = r.store_index and
req_tag = r.store_tag and
r.rows_valid(to_integer(req_row(ROW_LINEBITS-1 downto 0))) = '1' then
is_hit := '1';
hit_way := r.store_way;
end if;
if r.stalled_hit = '1' then
is_hit := '1';
hit_way := r.stalled_way;
end if;

-- Generate the "hit" and "miss" signals for the synchronous blocks
if i_in.req = '1' and access_ok = '1' and flush_in = '0' and rst = '0' then
@ -646,20 +684,22 @@ begin
-- I prefer not to do just yet as it would force fetch2 to know about
-- some of the cache geometry information.
--
insn := (others => '0');
icode := INSN_illegal;
if r.hit_valid = '1' then
assert not is_X(r.hit_way) severity failure;
if is_X(r.hit_way) then
insn := (others => 'X');
else
insn := read_insn_word(r.hit_nia, cache_out(to_integer(r.hit_way)));
-- Currently we use only the top bit for indicating illegal
-- instructions because we know that insn_codes fit into 9 bits.
if is_X(insn) then
insn := (others => '0');
elsif insn(ICWORDLEN - 1) = '0' then
icode := insn_code'val(to_integer(unsigned(insn(ICWORDLEN-1 downto INSN_IMAGE_BITS))));
insn(31 downto 26) := recode_primary_opcode(icode);
end if;
end if;
end if;
assert not (r.hit_valid = '1' and is_X(r.hit_way)) severity failure;
-- Currently we use only the top bit for indicating illegal
-- instructions because we know that insn_codes fit into 9 bits.
if is_X(insn) then
insn := (others => '0');
elsif insn(ICWORDLEN - 1) = '0' then
icode := insn_code'val(to_integer(unsigned(insn(ICWORDLEN-1 downto INSN_IMAGE_BITS))));
insn(31 downto 26) := recode_primary_opcode(icode);
end if;

i_out.insn <= insn(31 downto 0);
i_out.icode <= icode;
log_insn <= insn;
@ -672,7 +712,7 @@ begin
i_out.next_pred_ntaken <= r.pred_ntaken;

-- Stall fetch1 if we have a miss on cache or TLB or a protection fault
stall_out <= i_in.req and not (is_hit and access_ok);
stall_out <= i_in.req and not (is_hit and access_ok) and not flush_in;

-- Wishbone requests output (from the cache miss reload machine)
wishbone_out <= r.wb;
@ -684,9 +724,17 @@ begin
if rising_edge(clk) then
-- keep outputs to fetch2 unchanged on a stall
-- except that flush or reset sets valid to 0
if stall_in = '1' then
if rst = '1' or flush_in = '1' then
r.hit_valid <= '0';
if rst = '1' or flush_in = '1' then
r.hit_valid <= '0';
r.stalled_hit <= '0';
r.stalled_way <= to_unsigned(0, WAY_BITS);
elsif stall_in = '1' then
if r.state = CLR_TAG then
r.stalled_hit <= '0';
elsif req_is_hit = '1' then
-- if we have a hit while stalled, remember it
r.stalled_hit <= '1';
r.stalled_way <= req_hit_way;
end if;
else
-- On a hit, latch the request for the next cycle, when the BRAM data
@ -706,6 +754,7 @@ begin
" way:" & to_hstring(req_hit_way) &
" RA:" & to_hstring(real_addr);
end if;
r.stalled_hit <= '0';
end if;
if stall_in = '0' then
-- Send stop marks and NIA down regardless of validity
@ -726,7 +775,6 @@ begin
variable tagset : cache_tags_set_t;
variable tag : cache_tag_t;
variable snoop_addr : real_addr_t;
variable snoop_tag : cache_tag_t;
variable snoop_cache_tags : cache_tags_set_t;
variable replace_way : way_sig_t;
begin
@ -759,15 +807,14 @@ begin
snoop_valid <= wb_snoop_in.cyc and wb_snoop_in.stb and wb_snoop_in.we;
snoop_addr := addr_to_real(wb_to_addr(wb_snoop_in.adr));
snoop_index <= get_index(snoop_addr);
snoop_tag := get_tag(snoop_addr, '0');
snoop_tag <= get_tag(snoop_addr, '0');
snoop_hits <= (others => '0');

-- On the next cycle, match up tags with the snooped address
-- to see if any ways need to be invalidated
if snoop_valid = '1' then
if is_X(snoop_addr) then
report "metavalue in snoop_addr" severity FAILURE;
end if;
snoop_cache_tags := cache_tags(to_integer(get_index(snoop_addr)));
for i in way_t loop
tag := read_tag(i, snoop_cache_tags);
tag := snoop_tags_set(i);
-- Ignore endian bit in comparison
tag(TAG_BITS - 1) := '0';
if tag = snoop_tag then
@ -775,6 +822,7 @@ begin
end if;
end loop;
end if;
snoop_index2 <= snoop_index;

-- Process cache invalidations
if inval_in = '1' then
@ -783,12 +831,12 @@ begin
end loop;
r.store_valid <= '0';
else
-- Do invalidations from snooped stores to memory, one
-- cycle after the address appears on wb_snoop_in.
-- Do invalidations from snooped stores to memory,
-- two cycles after the address appears on wb_snoop_in.
for i in way_t loop
if snoop_hits(i) = '1' then
assert not is_X(snoop_index) severity failure;
cache_valids(to_integer(snoop_index))(i) <= '0';
assert not is_X(snoop_index2) severity failure;
cache_valids(to_integer(snoop_index2))(i) <= '0';
end if;
end loop;
end if;
@ -846,15 +894,6 @@ begin
assert not is_X(replace_way) severity failure;
cache_valids(to_integer(r.store_index))(to_integer(replace_way)) <= '0';

-- Store new tag in selected way
for i in 0 to NUM_WAYS-1 loop
if to_unsigned(i, WAY_BITS) = replace_way then
tagset := cache_tags(to_integer(r.store_index));
write_tag(i, tagset, r.store_tag);
cache_tags(to_integer(r.store_index)) <= tagset;
end if;
end loop;

r.state <= WAIT_ACK;
end if;


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