Add Sstc extension

This adds the stimecmp[h] CSRs. It is enabled by default.

The code is slightly unfortunately structured to handle CSRs that don't fit the standard permission checks. I added a TODO to improve it.

Co-authored-by: Tim Hutt <timothy.hutt@codasip.com>

c_emulator/riscv_platform.c

@@ -67,6 +67,11 @@ bool sys_enable_zicboz(unit u)
   return rv_enable_zicboz;
 }
 
+bool sys_enable_sstc(unit u)
+{
+  return rv_enable_sstc;
+}
+
 uint64_t sys_pmp_count(unit u)
 {
   return rv_pmp_count;

c_emulator/riscv_platform.h

@@ -12,6 +12,7 @@ bool sys_enable_vext(unit);
 bool sys_enable_bext(unit);
 bool sys_enable_zicbom(unit);
 bool sys_enable_zicboz(unit);
+bool sys_enable_sstc(unit);
 
 uint64_t sys_pmp_count(unit);
 uint64_t sys_pmp_grain(unit);

c_emulator/riscv_platform_impl.c

@@ -19,6 +19,7 @@ bool rv_enable_vext = true;
 bool rv_enable_bext = false;
 bool rv_enable_zicbom = false;
 bool rv_enable_zicboz = false;
+bool rv_enable_sstc = false;
 
 bool rv_enable_dirty_update = false;
 bool rv_enable_misaligned = false;

c_emulator/riscv_platform_impl.h

@@ -23,6 +23,7 @@ extern bool rv_enable_vext;
 extern bool rv_enable_bext;
 extern bool rv_enable_zicbom;
 extern bool rv_enable_zicboz;
+extern bool rv_enable_sstc;
 extern bool rv_enable_writable_misa;
 extern bool rv_enable_dirty_update;
 extern bool rv_enable_misaligned;

c_emulator/riscv_sim.c

@@ -58,6 +58,7 @@ enum {
   OPT_ENABLE_ZCB,
   OPT_ENABLE_ZICBOM,
   OPT_ENABLE_ZICBOZ,
+  OPT_ENABLE_SSTC,
   OPT_CACHE_BLOCK_SIZE,
 };
 
@@ -428,6 +429,10 @@ static int process_args(int argc, char **argv)
       fprintf(stderr, "enabling Zicboz extension.\n");
       rv_enable_zicboz = true;
       break;
+    case OPT_ENABLE_SSTC:
+      fprintf(stderr, "enabling Sstc extension.\n");
+      rv_enable_sstc = true;
+      break;
     case OPT_CACHE_BLOCK_SIZE:
       block_size_exp = ilog2(atol(optarg));
 

model/riscv_csr_begin.sail

@@ -99,6 +99,9 @@ mapping clause csr_name_map = 0x141  <-> "sepc"
 mapping clause csr_name_map = 0x142  <-> "scause"
 mapping clause csr_name_map = 0x143  <-> "stval"
 mapping clause csr_name_map = 0x144  <-> "sip"
+/* Sstc - supervisor timer register */
+mapping clause csr_name_map = 0x14D  <-> "stimecmp"
+mapping clause csr_name_map = 0x15D  <-> "stimecmph"
 /* supervisor protection and translation */
 mapping clause csr_name_map = 0x180  <-> "satp"
 /* supervisor envcfg */

model/riscv_insts_zicsr.sail

@@ -83,6 +83,8 @@ function clause read_CSR(0x141) = get_xret_target(Supervisor) & pc_alignment_mas
 function clause read_CSR(0x142) = scause.bits
 function clause read_CSR(0x143) = stval
 function clause read_CSR(0x144) = lower_mip(mip, mideleg).bits
+function clause read_CSR(0x14D) = stimecmp[sizeof(xlen) - 1 .. 0]
+function clause read_CSR(0x15D if xlen == 32) = stimecmp[63 .. 32]
 function clause read_CSR(0x180) = satp
 
 /* user mode counters */
@@ -169,6 +171,8 @@ function clause write_CSR(0x141, value) = { set_xret_target(Supervisor, value) }
 function clause write_CSR(0x142, value) = { scause.bits = value; scause.bits }
 function clause write_CSR(0x143, value) = { stval = value; stval }
 function clause write_CSR(0x144, value) = { mip = legalize_sip(mip, mideleg, value); mip.bits }
+function clause write_CSR(0x14D, value) = { stimecmp[(sizeof(xlen) - 1) .. 0] = value; stimecmp[sizeof(xlen) - 1 ..0] }
+function clause write_CSR((0x15D, value) if xlen == 32) = { stimecmp[63 ..32] = value; stimecmp[63 .. 32] }
 function clause write_CSR(0x180, value) = { satp = legalize_satp(cur_Architecture(), satp, value); satp }
 
 /* user mode: seed (entropy source). writes are ignored */

model/riscv_platform.sail

@@ -211,8 +211,17 @@ function clint_dispatch() -> unit = {
   if mtimecmp <=_u mtime then {
     if   get_config_print_platform()
     then print_platform(" clint timer pending at mtime " ^ BitStr(mtime));
-    mip[MTI] = 0b1
-  }
+    mip[MTI] = 0b1;
+  };
+  /* Sstc - supervisor timer register */
+  if extensionEnabled(Ext_Sstc) then {
+    mip[STI] = 0b0;
+    if stimecmp <=_u mtime then {
+      if   get_config_print_platform()
+      then print_platform(" supervisor timer pending at mtime " ^ BitStr(mtime));
+      mip[STI] = 0b1;
+    }
+  };
 }
 
 /* The rreg effect is due to checking mtime. */

model/riscv_sys_control.sail

@@ -85,6 +85,10 @@ function clause is_CSR_defined(0x142) = extensionEnabled(Ext_S) // scause
 function clause is_CSR_defined(0x143) = extensionEnabled(Ext_S) // stval
 function clause is_CSR_defined(0x144) = extensionEnabled(Ext_S) // sip
 
+/* Sstc : supervisor timer register */
+function clause is_CSR_defined(0x14D) = extensionEnabled(Ext_S) & extensionEnabled(Ext_Sstc) // stimecmp
+function clause is_CSR_defined(0x15D) = extensionEnabled(Ext_S) & extensionEnabled(Ext_Sstc) & xlen == 32 // stimecmph
+
 /* supervisor mode: address translation */
 function clause is_CSR_defined(0x180) = extensionEnabled(Ext_S) // satp
 
@@ -132,6 +136,15 @@ function check_Counteren(csr : csreg, p : Privilege) -> bool = {
   feature_enabled_for_priv(p, mcounteren.bits[index], scounteren.bits[index])
 }
 
+// Return true if the stimecmp[h] CSR is accessible OR the CSR is not stimecmp[h].
+function check_Stimecmp(csr : csreg, p : Privilege) -> bool = {
+  // Check if it is not stimecmp.
+  if csr != 0x14D & csr != 0x15D
+  then return true;
+
+  p == Machine | (p == Supervisor & mcounteren[TM] == 0b1 & menvcfg[STCE] == 0b1)
+}
+
 /* Seed may only be accessed if we are doing a write, and access has been
  * allowed in the current priv mode
  */
@@ -153,8 +166,12 @@ function check_seed_CSR (csr : csreg, p : Privilege, isWrite : bool) -> bool = {
 function check_CSR(csr : csreg, p : Privilege, isWrite : bool) -> bool =
     is_CSR_defined(csr)
   & check_CSR_access(csrAccess(csr), csrPriv(csr), p, isWrite)
+  // TODO: If we add `p` back to is_CSR_defined() we could move these three
+  // check_ functions back there. We should also rename is_CSR_defined()
+  // to is_CSR_accessible() or similar.
   & check_TVM_SATP(csr, p)
   & check_Counteren(csr, p)
+  & check_Stimecmp(csr, p)
   & check_seed_CSR(csr, p, isWrite)
 
 /* Reservation handling for LR/SC.

model/riscv_sys_regs.sail

@@ -116,6 +116,9 @@ val sys_enable_bext = pure "sys_enable_bext" : unit -> bool
 val sys_enable_zicbom = pure "sys_enable_zicbom" : unit -> bool
 val sys_enable_zicboz = pure "sys_enable_zicboz" : unit -> bool
 
+// Is the Sstc stimecmp extension supported.
+val sys_enable_sstc = pure "sys_enable_sstc" : unit -> bool
+
 /* This function allows an extension to veto a write to Misa
    if it would violate an alignment restriction on
    unsetting C. If it returns true the write will have no effect. */
@@ -313,11 +316,55 @@ register mip     : Minterrupts /* Pending */
 register mie     : Minterrupts /* Enabled */
 register mideleg : Minterrupts /* Delegation to S-mode */
 
+bitfield MEnvcfg : bits(64) = {
+  // Supervisor TimeCmp Extension
+  STCE   : 63,
+  // Page Based Memory Types Extension
+  PBMTE  : 62,
+  // Reserved WPRI bits.
+  wpri_1 : 61 .. 8,
+  // Cache Block Zero instruction Enable
+  CBZE   : 7,
+  // Cache Block Clean and Flush instruction Enable
+  CBCFE  : 6,
+  // Cache Block Invalidate instruction Enable
+  CBIE   : 5 .. 4,
+  // Reserved WPRI bits.
+  wpri_0 : 3 .. 1,
+  // Fence of I/O implies Memory
+  FIOM   : 0,
+}
+
+bitfield SEnvcfg : xlenbits = {
+  // Cache Block Zero instruction Enable
+  CBZE   : 7,
+  // Cache Block Clean and Flush instruction Enable
+  CBCFE  : 6,
+  // Cache Block Invalidate instruction Enable
+  CBIE   : 5 .. 4,
+  // Reserved WPRI bits.
+  wpri_0 : 3 .. 1,
+  // Fence of I/O implies Memory
+  FIOM   : 0,
+}
+
+register menvcfg : MEnvcfg
+register senvcfg : SEnvcfg
+
+/* Supervisor timecmp */
+enum clause extension = Ext_Sstc
+function clause extensionEnabled(Ext_Sstc) = sys_enable_sstc()
+
 function legalize_mip(o : Minterrupts, v : xlenbits) -> Minterrupts = {
   /* The only writable bits are the S-mode bits, and with the 'N'
    * extension, the U-mode bits. */
   let v = Mk_Minterrupts(v);
-  [o with SEI = v[SEI], STI = v[STI], SSI = v[SSI]]
+  [o with
+    SEI = v[SEI],
+    SSI = v[SSI],
+    // STI is read only if Sstc is enabled and STCE is set (it is equal to stimecmp <= mtime).
+    STI = if extensionEnabled(Ext_Sstc) & menvcfg[STCE] == 0b1 then o[STI] else v[STI],
+  ]
 }
 
 function legalize_mie(o : Minterrupts, v : xlenbits) -> Minterrupts = {
@@ -737,53 +784,21 @@ function read_seed_csr() -> xlenbits = {
 /* Writes to the seed CSR are ignored */
 function write_seed_csr () -> xlenbits = zeros()
 
-bitfield MEnvcfg : bits(64) = {
-  // Supervisor TimeCmp Extension
-  STCE   : 63,
-  // Page Based Memory Types Extension
-  PBMTE  : 62,
-  // Reserved WPRI bits.
-  wpri_1 : 61 .. 8,
-  // Cache Block Zero instruction Enable
-  CBZE   : 7,
-  // Cache Block Clean and Flush instruction Enable
-  CBCFE  : 6,
-  // Cache Block Invalidate instruction Enable
-  CBIE   : 5 .. 4,
-  // Reserved WPRI bits.
-  wpri_0 : 3 .. 1,
-  // Fence of I/O implies Memory
-  FIOM   : 0,
-}
-
-bitfield SEnvcfg : xlenbits = {
-  // Cache Block Zero instruction Enable
-  CBZE   : 7,
-  // Cache Block Clean and Flush instruction Enable
-  CBCFE  : 6,
-  // Cache Block Invalidate instruction Enable
-  CBIE   : 5 .. 4,
-  // Reserved WPRI bits.
-  wpri_0 : 3 .. 1,
-  // Fence of I/O implies Memory
-  FIOM   : 0,
-}
-
-register menvcfg : MEnvcfg
-register senvcfg : SEnvcfg
-
 function legalize_menvcfg(o : MEnvcfg, v : bits(64)) -> MEnvcfg = {
   let v = Mk_MEnvcfg(v);
-  let o = [o with FIOM = if sys_enable_writable_fiom() then v[FIOM] else 0b0];
-  // Other extensions are not implemented yet so all other fields are read only zero.
-  o
+  [o with
+    FIOM = if sys_enable_writable_fiom() then v[FIOM] else 0b0,
+    STCE = if extensionEnabled(Ext_Sstc) then v[STCE] else 0b0,
+    // Other extensions are not implemented yet so all other fields are read only zero.
+  ]
 }
 
 function legalize_senvcfg(o : SEnvcfg, v : xlenbits) -> SEnvcfg = {
   let v = Mk_SEnvcfg(v);
-  let o = [o with FIOM = if sys_enable_writable_fiom() then v[FIOM] else 0b0];
-  // Other extensions are not implemented yet so all other fields are read only zero.
-  o
+  [o with
+    FIOM = if sys_enable_writable_fiom() then v[FIOM] else 0b0,
+    // Other extensions are not implemented yet so all other fields are read only zero.
+  ];
 }
 
 // Return whether or not FIOM is currently active, based on the current
@@ -878,3 +893,6 @@ function get_vtype_vma() = decode_agtype(vtype[vma])
 
 val get_vtype_vta : unit -> agtype
 function get_vtype_vta() = decode_agtype(vtype[vta])
+
+/* Sstc : Supervisor Timer Register */
+register stimecmp : bits(64)