l1d_coherence.md

l1d coherence spec

1 interface

1.1 cache miss req intf

1.2 snoop req intf

1.2.1 channels

// ace5 snoop channels
  // snoop addr
input  logic              snp_req_if_acvalid_i,
output logic              snp_req_if_acready_o,
input  cache_mem_if_ac_t  snp_req_if_ac_i,
  // snoop resp
output logic              snp_resp_if_crvalid_o,
input  logic              snp_resp_if_crready_i,
output cache_mem_if_cr_t  snp_resp_if_cr_o,
  // snoop data
output logic              snp_resp_if_cdvalid_o,
input  logic              snp_resp_if_cdready_i,
output cache_mem_if_cd_t  snp_resp_if_cd_o,

1.2.2 definition

// ace5 snoop channels
  // snoop addr
typedef struct packed {
  // mem_tid_t acid;
  logic [L1D_STB_LINE_ADDR_SIZE-1:0]  acaddr;
  logic [3:0]              acsoop;
  logic [2:0]              acprot;
} cache_mem_if_ac_t;

  // snoop resp
typedef struct packed {
  logic WasUnique;
  logic IsShared;
  logic PassDirty;
  logic Error; // for ECC, not used
  logic DataTransfer;
} cr_crresp_t;

typedef struct packed {
  // mem_tid_t crid;
  cr_crresp_t crresp;
} cache_mem_if_cr_t;

  // snoop data
typedef struct packed {
  // mem_tid_t cdid;
  logic [L1D_BANK_LINE_DATA_SIZE-1:0] cddata;
  logic                               cdlast;
} cache_mem_if_cd_t;

2 micro architecture support

2.1 pipeline diagram

2.2 corner cases

Some detailed corner case specification for snoop.

2.2.1 snoop req conflict with pipeline

If set idx conflict with valid req in pipeline, stall snoop buffer until no conflict in pipeline, meanwhile backpress all incoming req at s0.

2.2.2 snoop req conflict with mshr

If set idx conflict with valid req in mshr:

• if the mshr req has handshaked with next level memory, stall snoop buffer until no conflict in mshr, meanwhile backpress all incoming req at s0;
• else if the mshr req has not hankshaked with next level memory, no snoop req confict.

2.2.3 snoop req conflict with lfb

No need to check as a valid entry in lfb has to be a valid entry in mshr.

2.2.4 snoop req conflict with ewrq

If set idx conflict with valid req in ewrq, wait until ewrq finish the evict, meanwhile backpress all incoming req at s0. Then do the snoop.

For the SCU or next level memory control, if there is a same-line-addr write back req from last level memory which hit a in flight coherent transaction, the write back cache line should be forwarded to the hitted coherence transaction, take it as the cohernce resp data, and no need to write it into the data ram.

3 protocol

Use MESI protocol, each cache line has one of four states, the cache lines' state is maintained at private cache's line state table(LST) or directory/snoopy filter in shared snoop control unit(SCU)

3.1 stable states

state/attribute	Valid	Dirty	Exclusive
I (Invalid)	N	N	-
SC(SharedClean)	Y	N	N
UC(UniqueClean)	Y	N	Y
UD(UniqueDirty)	Y	Y	Y

3.2 transient states

3.3 messages

3.3.1 request msg

msg type class	msg type	description
requestor req	ReadNotSharedDirty	Read request to a Snoopable address region to carry out a load from the cache line. Data must be provided to the Requester in UC, UD, or SC states only.
	ReadUnique
	ReadOnce
	CleanUnique	Request to a Snoopable address region to change the cache state at the Requester to Unique to carry out a store to the cache line.
evict req	Evict
	WriteBackFull
	WriteEvictFull
scu snoop req	SnpShared	Snoop request to obtain a copy of the cache line in Shared state while leaving any cached copy in Shared state. Must not leave the cache line in Unique state.
	SnpUnique	Snoop request to obtain a copy of the cache line in Unique state while invalidating any cached copies. Must change the cache line to Invalid state.
	SnpCleanInvalid	Snoop request to Invalidate the cache line at the Snoopee and obtain any Dirty copy, used in SF eviction.

3.3.2 dataless response msg

msg type class	msg type	description
responsor resp	Comp_I
	Comp_UC
	Comp_SC
writeback req resp	CompDBIDResp	CopyBack requests' completion response. Comp: WriteData transaction is done. DBIDResp: resources are available to accept the WriteData response
snoop resp	SnpAck	L1 send to L2 when take the forwarded snoop req from L2
	SnpResp_I
	SnpResp_SC
resp final ack	CompAck	Sent by the Requester on receipt of the Completion response

3.3.3 data msg

msg type class	msg type	description
responsor resp data	CompData_I
	CompData_UC
	CompData_SC
	CompData_UD_PD
copy back req data	CBWrData_UC
	CBWrData_SC
	CBWrData_UD_PD
	CBWrData_I
snoop resp data	SnpRespData_I
	SnpRespData_I_PD
	SnpRespData_SC
	SnpRespData_SC_PD
	SnpRespData_UC
	SnpRespData_UD

3.4 states transition

3.4.1 cpu ld/st req fsm

current state/[output/next state]/input	default next state	default output	cpu_req_load	cpu_req_store	scu_resp_data_UC	scu_resp_data_SC	scu_resp_data_UD	scu_resp_UD	scu_SnpShared	scu_SnpUnique	scu_SnpCleanInvalid	refill_evict
I	-	-	/I_LD_S1	/I_ST_S1	-	-	-	-	/I_SNP_SHARED_S1	/I_SNP_UNIQUE_S1	/I_SNP_CLEANINVALID_S1	-
I_LD_S1(tag, lst check)	I_LD_S2_MISS(miss)	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	-
I_LD_S2_MISS(new mshr)	I_LD_IN_MSHR_NOTSENT	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	-
I_LD_IN_MSHR_NOTSENT	I_LD_IN_MSHR_SENT	ReadNotSharedDirty	stall input	stall input	-	-	-	-	snp fsm run/I_LD_SNP_STALL	snp fsm run/I_LD_SNP_STALL	snp fsm run/I_LD_SNP_STALL	-
I_LD_IN_MSHR_SENT		-	stall input	stall input	CompAck/I_UC_DATA_REFILL	CompAck/I_SC_DATA_REFILL	-	-	stall input	stall input	stall input	-
I_UC_DATA_REFILL	UC	load_lsu_resp	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
I_SC_DATA_REFILL	SC	load_lsu_resp	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
I_UD_DATA_REFILL	UD	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
I_ST_S1(tag, lst check)	I_LD_S2_MISS(miss)	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	-
I_ST_S2_MISS(new mshr)	I_ST_IN_MSHR_NOTSENT	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	-
I_ST_IN_MSHR_NOTSENT	I_ST_IN_MSHR_SENT	SendReadUnique	stall input	stall input	-	-	-	-	snp fsm run/I_ST_SNP_STALL	snp fsm run/I_ST_SNP_STALL	snp fsm run/I_ST_SNP_STALL	-
I_ST_IN_MSHR_SENT		-	stall input	stall input	-	-	CompAck/I_UD_DATA_REFILL	-	stall input	stall input	stall input	-
I_SNP_SHARED_S1(tag, lst check)	I_SNP_x_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
I_SNP_UNIQUE_S1(tag, lst check)	I_SNP_x_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
I_SNP_CLEANINVALID_S1(tag, lst check)	I_SNP_x_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
I_SNP_x_S2(snp resp)	I	SnpResp_I	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
I_LD_SNP_STALL	I_LD_IN_MSHR_NOTSENT(when snp finish)	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
I_ST_SNP_STALL	I_ST_IN_MSHR_NOTSENT(when snp finish)	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
UC	-	-	/UC_LD_S1	/UC_ST_S1	-	-	-	-	/UC_SNP_SHARED_S1	/UC_SNP_UNIQUE_S1	/UC_SNP_CLEANINVALID_S1	/UC_EVICT_S1
UC_LD_S1(tag, lst check)	UC_LD_S2_HIT(hit)	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	stall evict
UC_LD_S2_HIT(hit resp)	UC	load_lsu_resp	stall input	stall input	-	-	-	-	snp fsm run	snp fsm run	snp fsm run	stall evict
UC_ST_S1(tag, lst check)	UC_ST_S2_HIT(hit)	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	stall evict
UC_ST_S2_HIT(wr data, tag ram, lst)	UD	-	stall input	stall input	-	-	-	-	snp fsm run	snp fsm run	snp fsm run	stall evict
UC_SNP_SHARED_S1(tag, lst check)	UC_SNP_SHARED_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UC_SNP_SHARED_S2(wr lst; snp resp)	SC	SnpResp_SC	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UC_SNP_UNIQUE_S1(tag, lst check)	UC_SNP_UNIQUE_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UC_SNP_UNIQUE_S2(wr lst; snp resp)	I	SnpResp_I	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UC_SNP_CLEANINVALID_S1(tag, lst check)	UC_SNP_CLEANINVALID_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UC_SNP_CLEANINVALID_S2(wr lst; snp resp)	I	SnpResp_I	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UC_EVICT_S1(rd tag, lst; choose way)	UC_EVICT_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
UC_EVICT_S2(wr lst)	I	Evict	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
SC	-	-	/SC_LD_S1	/SC_ST_S1	-	-	-	-	/SC_SNP_SHARED_S1	/SC_SNP_UNIQUE_S1	/SC_SNP_CLEANINVALID_S1	/SC_EVICT_S1
SC_LD_S1(tag, lst check)	SC_LD_S2_HIT(hit)	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	stall evict
SC_LD_S2_HIT(hit resp)	SC	load_lsu_resp	stall input	stall input	-	-	-	-	snp fsm run	snp fsm run	snp fsm run	stall evict
SC_ST_S1(tag, lst check)	SC_ST_S2_MISS(access)	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	stall evict
SC_ST_S2_MISS	SC_ST_IN_MSHR_NOTSENT	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	stall evict
SC_ST_IN_MSHR_NOTSENT	SC_ST_IN_MSHR_SENT	CleanUnique	stall input	stall input	-	-	-	-	snp fsm run/SC_ST_SNP_STALL	snp fsm run/SC_ST_SNP_STALL	snp fsm run/SC_ST_SNP_STALL	stall evict
SC_ST_IN_MSHR_SENT		-	stall input	stall input	-	-	-	CompAck/SC_UD_ACC_UPDATE	stall input	stall input	stall input	stall evict
SC_UD_ACC_UPDATE (wr data ram, lst)	UD	-	stall input	stall input	-	-	-	-	snp fsm run	snp fsm run	snp fsm run	stall evict
SC_SNP_SHARED_S1(tag, lst check)	SC_SNP_SHARED_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
SC_SNP_SHARED_S2(wr lst; snp resp)	SC	SnpResp_SC	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
SC_SNP_UNIQUE_S1(tag, lst check)	SC_SNP_UNIQUE_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
SC_SNP_UNIQUE_S2(wr lst; snp resp)	I	SnpResp_I	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
SC_SNP_CLEANINVALID_S1(tag, lst check)	SC_SNP_CLEANINVALID_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
SC_SNP_CLEANINVALID_S2(wr lst; snp resp)	I	SnpResp_I	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
SC_ST_SNP_STALL	SC_ST_IN_MSHR_NOTSENT(when snp finish)	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
SC_EVICT_S1(rd tag, lst; choose way)	SC_EVICT_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
SC_EVICT_S2(wr lst)	I	Evict	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
UD	-	-	/UD_LD_S1	/UD_ST_S1	-	-	-	-	/UD_SNP_SHARED_S1	/UD_SNP_UNIQUE_S1	/UD_SNP_CLEANINVALID_S1	/UD_WriteBack_S1
UD_LD_S1(tag, lst check)	UD_LD_S2_HIT(hit)	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	stall evict
UD_LD_S2_HIT(hit resp)	UD	load_lsu_resp	stall input	stall input	-	-	-	-	snp fsm run	snp fsm run	snp fsm run	stall evict
UD_ST_S1(tag, lst check)	UD_ST_S2_HIT(hit)	-	stall input	stall input	-	-	-	-	stall at snp buf	stall at snp buf	stall at snp buf	stall evict
UD_ST_S2_HIT(wr data, tag ram, lst)	UD	-	stall input	stall input	-	-	-	-	snp fsm run	snp fsm run	snp fsm run	stall evict
UD_SNP_SHARED_S1(tag, lst check)	UD_SNP_SHARED_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UD_SNP_SHARED_S2(wr lst; snp data resp)	SC	SnpRespData_SC_PD	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UD_SNP_UNIQUE_S1(tag, lst check)	UD_SNP_UNIQUE_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UD_SNP_UNIQUE_S2(wr lst; snp data resp)	I	SnpRespData_I_PD	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UD_SNP_CLEANINVALID_S1(tag, lst check)	UD_SNP_CLEANINVALID_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UD_SNP_CLEANINVALID_S2(wr lst; snp data resp)	I	SnpRespData_I_PD	stall input	stall input	-	-	-	-	stall input	stall input	stall input	stall evict
UD_WriteBack_S1(rd tag ram, lst; choose way)	UD_WriteBack_S2	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
UD_WriteBack_S2(rd data ram)	UD_WriteBack_S3	CBWrData_UD_PD	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-
UD_WriteBack_S3(wr lst)	I	-	stall input	stall input	-	-	-	-	stall input	stall input	stall input	-

3.4.2 scu snp req fsm

4 l1d snoop control

4.1 SnpShared

4.1.1 description

Snoop request to obtain a copy of the cache line in Shared state while leaving any cached copy in Shared state. Must not leave the cache line in Unique state.

4.1.2 fsm

• all (during the whole snoop transaction):

  • 1. stop l1d bank from receiving new st, ptw, ld req from core
  • 2. stall mlfb refill transaction [mlfb_cache_peek_valid, mlfb_cache_check_valid, mlfb_cache_evict_valid, mlfb_cache_evict_bypass, mlfb_cache_refill_valid] if no sent-out line addr hit in mshr(cond s0.3)

• s0:

  • 1. if s1 is a store hit need to write data ram, wait for 1 cycle
  • 2. check pipeline, mshr, ewrq for same line addr, if any hit:

    no.	line state	next stage
    s0.1	in mshr not sent out, last stable state: I (I->UC, I->SC, I->UD)	goto s1: read tag ram, read lst
    s0.2	in mshr not sent out, last stable state: SC(SC->UD)	goto s1: read tag ram, read lst
    s0.3	in mshr sent out	stall snp until mshr no conflict
    s0.4	in ewrq	stall snp until ewrq no conflict
    s0.5	in pipeline	stall snp until pipe no conflict
    s0.6	no conflict	goto s1: read tag ram, read lst

• s1:

  • 1. read tag ram, read lst, then goto s2

• s2:

  • 1. compare tag, check lst
  • 2. read data ram if needed(cond s2.3, s2.4, s2.5)
  • 3. write lst if needed(cond s2.4, s2.5)

  no.	line state	next stage
  s2.1	tag miss	goto s3: dataless resp: SnpResp_I
  s2.2	I	goto s3: dataless resp: SnpResp_I
  s2.3	SC	goto s3: data resp: SnpResp_SC
  s2.4	UC	goto s3: data resp: SnpResp_SC
  s2.5	UD	goto s3: data resp: SnpResp_SC_PD

• s3:

  • 1. dataless resp if needed(cond s2.1, s2.2), finish snoop transaction
  • 2. or data resp if needed(cond s2.3, s2.4, s2.5), finish snoop transaction

4.2 SnpUnique

4.2.1 description

Snoop request to obtain a copy of the cache line in Unique state while invalidating any cached copies. Must change the cache line to Invalid state.

4.2.2 fsm

• all (during the whole snoop transaction):

  • 1. stop l1d bank from receiving new st, ptw, ld req from core
  • 2. stall mlfb refill transaction [mlfb_cache_peek_valid, mlfb_cache_check_valid, mlfb_cache_evict_valid, mlfb_cache_evict_bypass, mlfb_cache_refill_valid] if no sent-out line addr hit in mshr(cond s0.3)

• s0:

  • 1. if s1 is a store hit need to write data ram, wait for 1 cycle
  • 2. check pipeline, mshr, ewrq for same line addr, if any hit:

    no.	line state	next stage
    s0.1	in mshr not sent out, last stable state: I (I->UC, I->SC, I->UD)	goto s1: read tag ram, read lst
    s0.2	in mshr not sent out, last stable state: SC(SC->UD)	goto s1: read tag ram, read lst
    s0.3	in mshr sent out	stall snp until mshr no conflict
    s0.4	in ewrq	stall snp until ewrq no conflict
    s0.5	in pipeline	stall snp until pipe no conflict
    s0.6	no conflict	goto s1: read tag ram, read lst

• s1:

  • 1. read tag ram, read lst if needed, then goto s2

• s2:

  • 1. compare tag, check lst
  • 2. read data ram if needed(cond s2.3, s2.4, s2.5)
  • 3. write lst if needed(cond s2.3, s2.4, s2.5)

  no.	line state	next stage
  s2.1	tag miss	goto s3: dataless resp: SnpResp_I
  s2.2	I	goto s3: dataless resp: SnpResp_I
  s2.3	SC	goto s3: data resp: SnpResp_I
  s2.4	UC	goto s3: data resp: SnpResp_I
  s2.5	UD	goto s3: data resp: SnpResp_I_PD

• s3:

  • 1. dataless resp if needed(cond s2.1, s2.2), finish snoop transaction
  • 2. or data resp if needed(cond s2.3, s2.4, s2.5), finish snoop transaction

4.3 SnpCleanInvalid

4.3.1 description

Snoop request to Invalidate the cache line at the Snoopee and obtain any Dirty copy. Might also be generated by the interconnect without a corresponding request. Must change the cache line to Invalid state.

4.3.2 fsm

• all (during the whole snoop transaction):

  • 1. stop l1d bank from receiving new st, ptw, ld req from core
  • 2. stall mlfb refill transaction [mlfb_cache_peek_valid, mlfb_cache_check_valid, mlfb_cache_evict_valid, mlfb_cache_evict_bypass, mlfb_cache_refill_valid] if no sent-out line addr hit in mshr(cond s0.3)

• s0:

  • 1. if s1 is a store hit need to write data ram, wait for 1 cycle
  • 2. check pipeline, mshr, ewrq for same line addr, if any hit:

    no.	line state	next stage
    s0.1	in mshr not sent out, last stable state: I (I->UC, I->SC, I->UD)	goto s1: read tag ram, read lst
    s0.2	in mshr not sent out, last stable state: SC(SC->UD)	goto s1: read tag ram, read lst
    s0.3	in mshr sent out	stall snp until mshr no conflict
    s0.4	in ewrq	stall snp until ewrq no conflict
    s0.5	in pipeline	stall snp until pipe no conflict
    s0.6	no conflict	goto s1: read tag ram, read lst

• s1:

  • 1. read tag ram, read lst if needed, then goto s2

• s2:

  • 1. compare tag, check lst
  • 2. read data ram if needed(cond s2.3, s2.4, s2.5)
  • 3. write lst if needed(cond s2.3, s2.4, s2.5)

  no.	line state	next stage
  s2.1	tag miss	goto s3: dataless resp: SnpResp_I
  s2.2	I	goto s3: dataless resp: SnpResp_I
  s2.3	SC	goto s3: dataless resp: SnpResp_I
  s2.4	UC	goto s3: dataless resp: SnpResp_I
  s2.5	UD	goto s3: data resp: SnpResp_I_PD

• s3:

  • 1. dataless resp if needed(cond s2.1, s2.2), finish snoop transaction
  • 2. or data resp if needed(cond s2.3, s2.4, s2.5), finish snoop transaction