fence

Memory ordering fence

This instruction is defined by:

Encoding

Synopsis

Orders memory operations.

The fence instruction is used to order device I/O and memory accesses as viewed by other RISC-V harts and external devices or coprocessors. Any combination of device input (I), device output (O), memory reads (R), and memory writes (W) may be ordered with respect to any combination of the same. Informally, no other RISC-V hart or external device can observe any operation in the successor set following a fence before any operation in the predecessor set preceding the fence.

The predecessor and successor fields have the same format to specify operation types:

`pred`				`succ`
27	26	25	24	23	22	21	20
PI	PO	PR	PW	SI	SO	SR	SW

Table 1. Fence mode encoding
fm field	Mnemonic	Meaning
0000	none	Normal Fence
1000	TSO	With `FENCE RW,RW`: exclude write-to-read ordering; otherwise: Reserved for future use.
other		Reserved for future use.

When the mode field fm is 0001 and both the predecessor and successor sets are 'RW', then the instruction acts as a special-case fence.tso. fence.tso orders all load operations in its predecessor set before all memory operations in its successor set, and all store operations in its predecessor set before all store operations in its successor set. This leaves non-AMO store operations in the 'fence.tso’s predecessor set unordered with non-AMO loads in its successor set.

When mode field fm is not 0001, or when mode field fm is 0001 but the pred and succ fields are not both 'RW' (0x3), then the fence acts as a baseline fence (e.g., fm is effectively 0000). This is unaffected by the FIOM bits, described below (implicit promotion does not change how fence.tso is decoded).

The rs1 and rd fields are unused and ignored.

In modes other than M-mode, fence is further affected by menvcfg.FIOM, senvcfg.FIOM<% if ext?(:H) %>, and/or henvcfg.FIOM<% end %> as follows:

Table 2. Effective PR/PW/SR/SW in (H)S-mode
menvcfg.FIOM	`pred.PI` `pred.PO` `succ.SI` `succ.SO`	effective `PR` effective `PW` effective `SR` effective `SW`
0	-	from encoding
1	0	from encoding
1	1	1

Table 3. Effective PR/PW/SR/SW in U-mode
menvcfg.FIOM	senvcfg.FIOM	`pred.PI` `pred.PO` `succ.SI` `succ.SO`	effective `PR` effective `PW` effective `SR` effective `SW`
0	0	-	from encoding
0	1	0	from encoding
0	1	1	1
1	-	0	from encoding
1	-	1	1

<%- if ext?(:H) -%> .Effective PR/PW/SR/SW in VS-mode and VU-mode

menvcfg.FIOM	henvcfg.FIOM	`pred.PI` `pred.PO` `succ.SI` `succ.SO`	effective `PR` effective `PW` effective `SR` effective `SW`
0	0	-	from encoding
0	1	0	from encoding
0	1	1	1
1	-	0	from encoding
1	-	1	1

<%- end -%>

Access

Always

Decode Variables

Bits<4> fm = $encoding[31:28];
Bits<4> pred = $encoding[27:24];
Bits<4> succ = $encoding[23:20];
Bits<5> rs1 = $encoding[19:15];
Bits<5> rd = $encoding[11:7];

Execution

Pruned, XLEN == 64
Original

Boolean is_fence_tso;
Boolean is_pause;
if (fm == 1) {
  if (pred == 0x3 && succ == 0x3) {
    is_fence_tso = true;
  }
}

Boolean pred_i = pred[3] == 1;
Boolean pred_o = pred[2] == 1;
Boolean pred_r = pred[1] == 1;
Boolean pred_w = pred[0] == 1;
Boolean succ_i = succ[3] == 1;
Boolean succ_o = succ[2] == 1;
Boolean succ_r = succ[1] == 1;
Boolean succ_w = succ[0] == 1;
fence_tso();

Boolean is_fence_tso;
Boolean is_pause;
if (fm == 1) {
  if (pred == 0x3 && succ == 0x3) {
    is_fence_tso = true;
  }
}
if (implemented?(ExtensionName::Zihintpause)) {
  if ((pred == 1) && (succ == 0) && (fm == 0) && (rd == 0) && (rs1 == 0)) {
    is_pause = true;
  }
}
Boolean pred_i = pred[3] == 1;
Boolean pred_o = pred[2] == 1;
Boolean pred_r = pred[1] == 1;
Boolean pred_w = pred[0] == 1;
Boolean succ_i = succ[3] == 1;
Boolean succ_o = succ[2] == 1;
Boolean succ_r = succ[1] == 1;
Boolean succ_w = succ[0] == 1;
if (is_fence_tso) {
  fence_tso();
} else if (is_pause) {
  pause();
} else {
  if (mode() == PrivilegeMode::S) {
    if (CSR[menvcfg].FIOM == 1) {
      if (pred_i) {
        pred_r = true;
      }
      if (pred_o) {
        pred_w = true;
      }
      if (succ_i) {
        succ_r = true;
      }
      if (succ_o) {
        succ_w = true;
      }
    }
  } else if (mode() == PrivilegeMode::U) {
    if ((CSR[menvcfg].FIOM | CSR[senvcfg].FIOM) == 1) {
      if (pred_i) {
        pred_r = true;
      }
      if (pred_o) {
        pred_w = true;
      }
      if (succ_i) {
        succ_r = true;
      }
      if (succ_o) {
        succ_w = true;
      }
    }
  } else if (mode() == PrivilegeMode::VS || mode() == PrivilegeMode::VU) {
    if ((CSR[menvcfg].FIOM | CSR[henvcfg].FIOM) == 1) {
      if (pred_i) {
        pred_r = true;
      }
      if (pred_o) {
        pred_w = true;
      }
      if (succ_i) {
        succ_r = true;
      }
      if (succ_o) {
        succ_w = true;
      }
    }
  }
  fence(pred_i, pred_o, pred_r, pred_w, succ_i, succ_o, succ_r, succ_w);
}