Handle address translation for misaligned loads and stores better

Refactor the LOAD and STORE instruction so they split misaligned
accesses into multiple sub-accesses and perform address translation
separately. This means we should handle the case where a misaligned
access straddles a page boundary in a sensible way, even if we don't
yet cover the full range of possibilities allowed for any RISC-V
implementation.

There are options for the order in which misaligned happen, i.e. from
high-to-low or from low-to-high as well as the granularity of the splitting,
either all the way to bytes or to the largest aligned size.

In addition tidy up the implementation in a few ways:

- Very long lines on the LOAD encdec were fixed by adding a helper

- Add some linebreaks in the code so it reads as less claustrophobic

- Ensure we use the same names for arguments in encdec/execute/assembly.
  Previously we used 'size' and 'width'. I opted for 'width' consistently.

Makefile

@@ -93,6 +93,7 @@ SAIL_VM_SRCS += riscv_vmem_pte.sail
 SAIL_VM_SRCS += riscv_vmem_ptw.sail
 SAIL_VM_SRCS += riscv_vmem_tlb.sail
 SAIL_VM_SRCS += riscv_vmem.sail
+SAIL_VM_SRCS += riscv_vmem_utils.sail
 
 # Non-instruction sources
 PRELUDE = prelude.sail $(SAIL_XLEN) $(SAIL_FLEN) $(SAIL_VLEN) prelude_mem_metadata.sail prelude_mem.sail

c_emulator/riscv_platform.c

@@ -62,6 +62,16 @@ bool sys_enable_bext(unit u)
   return rv_enable_bext;
 }
 
+bool sys_misaligned_order_decreasing(unit u)
+{
+  return rv_misaligned_order_decreasing;
+}
+
+bool sys_misaligned_to_byte(unit u)
+{
+  return rv_misaligned_to_byte;
+}
+
 uint64_t sys_pmp_count(unit u)
 {
   return rv_pmp_count;

c_emulator/riscv_platform.h

@@ -12,6 +12,9 @@ bool sys_enable_writable_fiom(unit);
 bool sys_enable_vext(unit);
 bool sys_enable_bext(unit);
 
+bool sys_misaligned_order_decreasing(unit);
+bool sys_misaligned_to_byte(unit);
+
 uint64_t sys_pmp_count(unit);
 uint64_t sys_pmp_grain(unit);
 

c_emulator/riscv_platform_impl.c

@@ -6,6 +6,10 @@
 uint64_t rv_pmp_count = 0;
 uint64_t rv_pmp_grain = 0;
 
+/* Defaults for misaligned access behavior in the generated simulator */
+bool rv_misaligned_order_decreasing = false;
+bool rv_misaligned_to_byte = false;
+
 bool rv_enable_svinval = false;
 bool rv_enable_zcb = false;
 bool rv_enable_zfinx = false;

c_emulator/riscv_platform_impl.h

@@ -11,6 +11,9 @@
 extern uint64_t rv_pmp_count;
 extern uint64_t rv_pmp_grain;
 
+extern bool rv_misaligned_order_decreasing;
+extern bool rv_misaligned_to_byte;
+
 extern bool rv_enable_svinval;
 extern bool rv_enable_zcb;
 extern bool rv_enable_zfinx;

c_emulator/riscv_sim.c

@@ -56,6 +56,8 @@ enum {
   OPT_PMP_GRAIN,
   OPT_ENABLE_SVINVAL,
   OPT_ENABLE_ZCB,
+  OPT_MISALIGNED_ORDER_DEC,
+  OPT_MISALIGNED_TO_BYTE,
 };
 
 static bool do_dump_dts = false;
@@ -127,6 +129,8 @@ static struct option options[] = {
     {"enable-misaligned",           no_argument,       0, 'm'                     },
     {"pmp-count",                   required_argument, 0, OPT_PMP_COUNT           },
     {"pmp-grain",                   required_argument, 0, OPT_PMP_GRAIN           },
+    {"misaligned-order-decreasing", no_argument,       0, OPT_MISALIGNED_ORDER_DEC},
+    {"misaligned-to-byte",          no_argument,       0, OPT_MISALIGNED_TO_BYTE  },
     {"enable-next",                 no_argument,       0, 'N'                     },
     {"ram-size",                    required_argument, 0, 'z'                     },
     {"disable-compressed",          no_argument,       0, 'C'                     },
@@ -315,6 +319,18 @@ static int process_args(int argc, char **argv)
       }
       rv_pmp_grain = pmp_grain;
       break;
+    case OPT_MISALIGNED_ORDER_DEC:
+      fprintf(stderr,
+              "misaligned access virtual addresses will be translated in "
+              "decreasing order.\n");
+      rv_misaligned_order_decreasing = true;
+      break;
+    case OPT_MISALIGNED_TO_BYTE:
+      fprintf(stderr,
+              "misaligned accesses will be split into individual "
+              "byte operations.\n");
+      rv_misaligned_to_byte = true;
+      break;
     case 'C':
       fprintf(stderr, "disabling RVC compressed instructions.\n");
       rv_enable_rvc = false;

model/prelude.sail

@@ -10,6 +10,7 @@ default Order dec
 
 $include <smt.sail>
 $include <option.sail>
+$include <result.sail>
 $include <arith.sail>
 $include <string.sail>
 $include "mapping.sail"

model/riscv_insts_base.sail

@@ -299,8 +299,8 @@ union clause ast = LOAD : (bits(12), regidx, regidx, bool, word_width, bool, boo
 
 /* unsigned loads are only present for widths strictly less than xlen,
    signed loads also present for widths equal to xlen */
-mapping clause encdec = LOAD(imm, rs1, rd, is_unsigned, size, false, false) if (size_bytes(size) < sizeof(xlen_bytes)) | (not(is_unsigned) & size_bytes(size) <= sizeof(xlen_bytes))
-  <-> imm @ rs1 @ bool_bits(is_unsigned) @ size_enc(size) @ rd @ 0b0000011 if (size_bytes(size) < sizeof(xlen_bytes)) | (not(is_unsigned) & size_bytes(size) <= sizeof(xlen_bytes))
+function valid_load_encdec(width : word_width, is_unsigned : bool) -> bool =
+  (size_bytes(width) < sizeof(xlen_bytes)) | (not(is_unsigned) & size_bytes(width) <= sizeof(xlen_bytes))
 
 val extend_value : forall 'n, 0 < 'n <= xlen. (bool, bits('n)) -> xlenbits
 function extend_value(is_unsigned, value) = if is_unsigned then zero_extend(value) else sign_extend(value)
@@ -313,9 +313,8 @@ function is_aligned(vaddr : xlenbits, width : word_width) -> bool =
     DOUBLE => vaddr[2..0] == zeros(),
   }
 
-// Return true if the address is misaligned and we don't support misaligned access.
-function check_misaligned(vaddr : xlenbits, width : word_width) -> bool =
-  not(plat_enable_misaligned_access()) & not(is_aligned(vaddr, width))
+mapping clause encdec = LOAD(imm, rs1, rd, is_unsigned, width, false, false) if valid_load_encdec(width, is_unsigned)
+  <-> imm @ rs1 @ bool_bits(is_unsigned) @ size_enc(width) @ rd @ 0b0000011 if valid_load_encdec(width, is_unsigned)
 
 function clause execute(LOAD(imm, rs1, rd, is_unsigned, width, aq, rl)) = {
   let offset : xlenbits = sign_extend(imm);
@@ -326,21 +325,30 @@ function clause execute(LOAD(imm, rs1, rd, is_unsigned, width, aq, rl)) = {
 
   /* Get the address, X(rs1) + offset.
      Some extensions perform additional checks on address validity. */
-  match ext_data_get_addr(rs1, offset, Read(Data), width_bytes) {
-    Ext_DataAddr_Error(e)  => { ext_handle_data_check_error(e); RETIRE_FAIL },
+  match ext_data_get_addr(rs1, offset, Read(Data), size_bytes(width)) {
+    Ext_DataAddr_Error(e)  => {
+      ext_handle_data_check_error(e);
+      RETIRE_FAIL
+    },
+
     Ext_DataAddr_OK(vaddr) => {
-      if   check_misaligned(vaddr, width)
-      then { handle_mem_exception(vaddr, E_Load_Addr_Align()); RETIRE_FAIL }
-      else match translateAddr(vaddr, Read(Data)) {
-        TR_Failure(e, _) => { handle_mem_exception(vaddr, e); RETIRE_FAIL },
-        TR_Address(paddr, _) =>
-
-          match mem_read(Read(Data), paddr, width_bytes, aq, rl, false) {
-            MemValue(result) => { X(rd) = extend_value(is_unsigned, result); RETIRE_SUCCESS },
-            MemException(e) => { handle_mem_exception(vaddr, e); RETIRE_FAIL },
-          },
+      if check_misaligned(vaddr, width) then {
+         handle_mem_exception(vaddr, E_Load_Addr_Align());
+         return RETIRE_FAIL
+      };
+
+      match vmem_read(vaddr, width_bytes, aq, rl, false) {
+         Err(vaddr, e) => {
+           handle_mem_exception(vaddr, e);
+           RETIRE_FAIL
+         },
+
+         Ok(data) => {
+           X(rd) = extend_value(is_unsigned, data);
+           RETIRE_SUCCESS
+         }
       }
-    },
+    }
   }
 }
 
@@ -362,17 +370,15 @@ mapping maybe_u = {
   false <-> ""
 }
 
-mapping clause assembly = LOAD(imm, rs1, rd, is_unsigned, size, aq, rl)
-  <-> "l" ^ size_mnemonic(size) ^ maybe_u(is_unsigned) ^ maybe_aq(aq) ^ maybe_rl(rl) ^ spc() ^ reg_name(rd) ^ sep() ^ hex_bits_signed_12(imm) ^ "(" ^ reg_name(rs1) ^ ")"
+mapping clause assembly = LOAD(imm, rs1, rd, is_unsigned, width, aq, rl)
+  <-> "l" ^ size_mnemonic(width) ^ maybe_u(is_unsigned) ^ maybe_aq(aq) ^ maybe_rl(rl) ^ spc() ^ reg_name(rd) ^ sep() ^ hex_bits_signed_12(imm) ^ "(" ^ reg_name(rs1) ^ ")"
 
 /* ****************************************************************** */
 union clause ast = STORE : (bits(12), regidx, regidx, word_width, bool, bool)
 
-mapping clause encdec = STORE(imm7 @ imm5, rs2, rs1, size, false, false)              if size_bytes(size) <= sizeof(xlen_bytes)
-  <-> imm7 : bits(7) @ rs2 @ rs1 @ 0b0 @ size_enc(size) @ imm5 : bits(5) @ 0b0100011 if size_bytes(size) <= sizeof(xlen_bytes)
+mapping clause encdec = STORE(imm7 @ imm5, rs2, rs1, width, false, false) if size_bytes(width) <= sizeof(xlen_bytes)
+  <-> imm7 : bits(7) @ rs2 @ rs1 @ 0b0 @ size_enc(width) @ imm5 : bits(5) @ 0b0100011 if size_bytes(width) <= sizeof(xlen_bytes)
 
-/* NOTE: Currently, we only EA if address translation is successful.
-   This may need revisiting. */
 function clause execute (STORE(imm, rs2, rs1, width, aq, rl)) = {
   let offset : xlenbits = sign_extend(imm);
   let width_bytes = size_bytes(width);
@@ -382,33 +388,31 @@ function clause execute (STORE(imm, rs2, rs1, width, aq, rl)) = {
 
   /* Get the address, X(rs1) + offset.
      Some extensions perform additional checks on address validity. */
-  match ext_data_get_addr(rs1, offset, Write(Data), width_bytes) {
-    Ext_DataAddr_Error(e)  => { ext_handle_data_check_error(e); RETIRE_FAIL },
-    Ext_DataAddr_OK(vaddr) =>
-      if   check_misaligned(vaddr, width)
-      then { handle_mem_exception(vaddr, E_SAMO_Addr_Align()); RETIRE_FAIL }
-      else match translateAddr(vaddr, Write(Data)) {
-        TR_Failure(e, _)    => { handle_mem_exception(vaddr, e); RETIRE_FAIL },
-        TR_Address(paddr, _) => {
-          let eares = mem_write_ea(paddr, width_bytes, aq, rl, false);
-          match (eares) {
-            MemException(e) => { handle_mem_exception(vaddr, e); RETIRE_FAIL },
-            MemValue(_) => {
-              let rs2_val = X(rs2);
-              match mem_write_value(paddr, width_bytes, rs2_val[width_bytes * 8 - 1 .. 0], aq, rl, false) {
-                MemValue(true)  => RETIRE_SUCCESS,
-                MemValue(false) => internal_error(__FILE__, __LINE__, "store got false from mem_write_value"),
-                MemException(e) => { handle_mem_exception(vaddr, e); RETIRE_FAIL }
-              }
-            }
-          }
+  match ext_data_get_addr(rs1, offset, Write(Data), size_bytes(width)) {
+    Ext_DataAddr_Error(e)  => {
+      ext_handle_data_check_error(e);
+      RETIRE_FAIL
+    },
+
+    Ext_DataAddr_OK(vaddr) => {
+      if check_misaligned(vaddr, width) then {
+        handle_mem_exception(vaddr, E_SAMO_Addr_Align());
+        return RETIRE_FAIL
+      };
+
+      match vmem_write_from_register(vaddr, width_bytes, unsigned(rs2), aq, rl, false) {
+        Ok() => RETIRE_SUCCESS,
+        Err(vaddr, e) => {
+          handle_mem_exception(vaddr, e);
+          return RETIRE_FAIL
         }
       }
+    }
   }
 }
 
-mapping clause assembly = STORE(imm, rs2, rs1, size, aq, rl)
-  <-> "s" ^ size_mnemonic(size) ^ maybe_aq(aq) ^ maybe_rl(rl) ^ spc() ^ reg_name(rs2) ^ sep() ^ hex_bits_signed_12(imm) ^ opt_spc() ^ "(" ^ opt_spc() ^ reg_name(rs1) ^ opt_spc() ^ ")"
+mapping clause assembly = STORE(imm, rs2, rs1, width, aq, rl)
+  <-> "s" ^ size_mnemonic(width) ^ maybe_aq(aq) ^ maybe_rl(rl) ^ spc() ^ reg_name(rs2) ^ sep() ^ hex_bits_signed_12(imm) ^ opt_spc() ^ "(" ^ opt_spc() ^ reg_name(rs1) ^ opt_spc() ^ ")"
 
 /* ****************************************************************** */
 union clause ast = ADDIW : (bits(12), regidx, regidx)

model/riscv_mem.sail

@@ -38,6 +38,14 @@
 function is_aligned_addr forall 'n. (addr : xlenbits, width : int('n)) -> bool =
   unsigned(addr) % width == 0
 
+function check_misaligned(vaddr : xlenbits, width : word_width) -> bool = {
+  if plat_enable_misaligned_access() then {
+    false
+  } else {
+    not(is_aligned_addr(vaddr, size_bytes(width)))
+  }
+}
+
 function read_kind_of_flags (aq : bool, rl : bool, res : bool) -> option(read_kind) =
   match (aq, rl, res) {
     (false, false, false) => Some(Read_plain),