+++ /dev/null
-from nmigen import Signal, Module, Cat, Const, Elaboratable
-from math import log2
-
-from ptw import TLBUpdate, PTE, ASID_WIDTH
-
-class PLRU(Elaboratable):
- """ PLRU - Pseudo Least Recently Used Replacement
-
- PLRU-tree indexing:
- lvl0 0
- / \
- / \
- lvl1 1 2
- / \ / \
- lvl2 3 4 5 6
- / \ /\/\ /\
- ... ... ... ...
- """
- def __init__(self, entries):
- self.entries = entries
- self.lu_hit = Signal(entries)
- self.replace_en_o = Signal(entries)
- self.lu_access_i = Signal()
- # Tree (bit per entry)
- self.TLBSZ = 2*(self.entries-1)
- self.plru_tree = Signal(self.TLBSZ)
- self.plru_tree_o = Signal(self.TLBSZ)
-
- def elaborate(self, platform):
- m = Module()
-
- # Just predefine which nodes will be set/cleared
- # E.g. for a TLB with 8 entries, the for-loop is semantically
- # equivalent to the following pseudo-code:
- # unique case (1'b1)
- # lu_hit[7]: plru_tree[0, 2, 6] = {1, 1, 1};
- # lu_hit[6]: plru_tree[0, 2, 6] = {1, 1, 0};
- # lu_hit[5]: plru_tree[0, 2, 5] = {1, 0, 1};
- # lu_hit[4]: plru_tree[0, 2, 5] = {1, 0, 0};
- # lu_hit[3]: plru_tree[0, 1, 4] = {0, 1, 1};
- # lu_hit[2]: plru_tree[0, 1, 4] = {0, 1, 0};
- # lu_hit[1]: plru_tree[0, 1, 3] = {0, 0, 1};
- # lu_hit[0]: plru_tree[0, 1, 3] = {0, 0, 0};
- # default: begin /* No hit */ end
- # endcase
- LOG_TLB = int(log2(self.entries))
- print(LOG_TLB)
- for i in range(self.entries):
- # we got a hit so update the pointer as it was least recently used
- hit = Signal(reset_less=True)
- m.d.comb += hit.eq(self.lu_hit[i] & self.lu_access_i)
- with m.If(hit):
- # Set the nodes to the values we would expect
- for lvl in range(LOG_TLB):
- idx_base = (1<<lvl)-1
- # lvl0 <=> MSB, lvl1 <=> MSB-1, ...
- shift = LOG_TLB - lvl;
- new_idx = Const(~((i >> (shift-1)) & 1), (1, False))
- plru_idx = idx_base + (i >> shift)
- print ("plru", i, lvl, hex(idx_base),
- plru_idx, shift, new_idx)
- m.d.comb += self.plru_tree_o[plru_idx].eq(new_idx)
-
- # Decode tree to write enable signals
- # Next for-loop basically creates the following logic for e.g.
- # an 8 entry TLB (note: pseudo-code obviously):
- # replace_en[7] = &plru_tree[ 6, 2, 0]; #plru_tree[0,2,6]=={1,1,1}
- # replace_en[6] = &plru_tree[~6, 2, 0]; #plru_tree[0,2,6]=={1,1,0}
- # replace_en[5] = &plru_tree[ 5,~2, 0]; #plru_tree[0,2,5]=={1,0,1}
- # replace_en[4] = &plru_tree[~5,~2, 0]; #plru_tree[0,2,5]=={1,0,0}
- # replace_en[3] = &plru_tree[ 4, 1,~0]; #plru_tree[0,1,4]=={0,1,1}
- # replace_en[2] = &plru_tree[~4, 1,~0]; #plru_tree[0,1,4]=={0,1,0}
- # replace_en[1] = &plru_tree[ 3,~1,~0]; #plru_tree[0,1,3]=={0,0,1}
- # replace_en[0] = &plru_tree[~3,~1,~0]; #plru_tree[0,1,3]=={0,0,0}
- # For each entry traverse the tree. If every tree-node matches
- # the corresponding bit of the entry's index, this is
- # the next entry to replace.
- replace = []
- for i in range(self.entries):
- en = []
- for lvl in range(LOG_TLB):
- idx_base = (1<<lvl)-1
- # lvl0 <=> MSB, lvl1 <=> MSB-1, ...
- shift = LOG_TLB - lvl;
- new_idx = (i >> (shift-1)) & 1;
- plru_idx = idx_base + (i>>shift)
- plru = Signal(reset_less=True,
- name="plru-%d-%d-%d" % (i, lvl, plru_idx))
- m.d.comb += plru.eq(self.plru_tree[plru_idx])
- # en &= plru_tree_q[idx_base + (i>>shift)] == new_idx;
- if new_idx:
- en.append(~plru) # yes inverted (using bool())
- else:
- en.append(plru) # yes inverted (using bool())
- print ("plru", i, en)
- # boolean logic manipulation:
- # plru0 & plru1 & plru2 == ~(~plru0 | ~plru1 | ~plru2)
- replace.append(~Cat(*en).bool())
- m.d.comb += self.replace_en_o.eq(Cat(*replace))
-
- return m
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