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: Ved Shanbhogue <[email protected]>

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_platform.sail

@@ -121,8 +121,13 @@ function within_htif_readable forall 'n, 0 < 'n <= max_mem_access . (addr : xlen
 
 val plat_insns_per_tick = pure {interpreter: "Platform.insns_per_tick", c: "plat_insns_per_tick", lem: "plat_insns_per_tick"} : unit -> int
 
-// assumes a single hart, since this typically is a vector of per-hart registers.
-register mtimecmp : bits(64)  // memory-mapped internal clint register.
+// Each hart has a memory-mapped mtimecmp register. Typically these are
+// exposed as an array in CLINT. The CLINT implementation here is currently
+// hard-coded to use the mtimecmp for hart 0.
+register mtimecmp : bits(64)
+
+// Unlike mtimecmp, stimecmp is a real CSR; not memory mapped.
+register stimecmp : bits(64)
 
 /* CLINT memory-mapped IO */
 
@@ -211,8 +216,17 @@ 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. */
@@ -461,13 +475,4 @@ function handle_illegal() -> unit = {
 }
 
 /* Platform-specific wait-for-interrupt */
-
-function platform_wfi() -> unit = {
-  /* speed execution by getting the timer to fire at the next instruction,
-   * since we currently don't have any other devices raising interrupts.
-   */
-  if mtime <_u mtimecmp then {
-    mtime  = mtimecmp;
-    mcycle = mtimecmp;
-  }
-}
+function platform_wfi() -> unit = ()

model/riscv_sstc.sail

@@ -0,0 +1,20 @@
+/*=======================================================================================*/
+/*  This Sail RISC-V architecture model, comprising all files and                        */
+/*  directories except where otherwise noted is subject the BSD                          */
+/*  two-clause license in the LICENSE file.                                              */
+/*                                                                                       */
+/*  SPDX-License-Identifier: BSD-2-Clause                                                */
+/*=======================================================================================*/
+
+/* Sstc - supervisor time compare */
+mapping clause csr_name_map = 0x14D  <-> "stimecmp"
+mapping clause csr_name_map = 0x15D  <-> "stimecmph"
+
+function clause is_CSR_defined(0x14D) = extensionEnabled(Ext_S) & extensionEnabled(Ext_Sstc)
+function clause is_CSR_defined(0x15D) = extensionEnabled(Ext_S) & extensionEnabled(Ext_Sstc) & xlen == 32
+
+function clause read_CSR(0x14D) = stimecmp[xlen - 1 .. 0]
+function clause read_CSR(0x15D if xlen == 32) = stimecmp[63 .. 32]
+
+function clause write_CSR(0x14D, value) = { stimecmp[(xlen - 1) .. 0] = value; stimecmp[xlen - 1 ..0] }
+function clause write_CSR((0x15D, value) if xlen == 32) = { stimecmp[63 ..32] = value; stimecmp[63 .. 32] }

model/riscv_sys_control.sail

@@ -32,14 +32,21 @@ function feature_enabled_for_priv(p : Privilege, machine_enable_bit : bit, super
 // Return true if the counter is enabled OR the CSR is not a counter.
 function check_Counteren(csr : csreg, p : Privilege) -> bool = {
   // Check if it is not a counter.
-  if csr <_u 0xC00 | 0xC1F <_u csr
-  then return true;
+  if csr <_u 0xC00 | 0xC1F <_u csr then return true;
 
   // Check the relevant bit in m/scounteren.
   let index = unsigned(csr[4 .. 0]);
   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
  */
@@ -61,8 +68,12 @@ 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

@@ -114,6 +114,13 @@ 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
+
+// Supervisor timecmp
+enum clause extension = Ext_Sstc
+function clause extensionEnabled(Ext_Sstc) = sys_enable_sstc()
+
 /* 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. */
@@ -308,6 +315,87 @@ function in32BitMode() -> bool = {
   cur_Architecture() == RV32
 }
 
+// envcfg Resisters
+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,
+}
+
+function legalize_menvcfg(o : MEnvcfg, v : bits(64)) -> MEnvcfg = {
+  let v = Mk_MEnvcfg(v);
+  [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);
+  [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.
+  ];
+}
+
+register menvcfg : MEnvcfg
+register senvcfg : SEnvcfg
+
+mapping clause csr_name_map = 0x30A  <-> "menvcfg"
+mapping clause csr_name_map = 0x31A  <-> "menvcfgh"
+mapping clause csr_name_map = 0x10A  <-> "senvcfg"
+
+function clause is_CSR_defined(0x30A) = extensionEnabled(Ext_U) // menvcfg
+function clause is_CSR_defined(0x31A) = extensionEnabled(Ext_U) & (xlen == 32) // menvcfgh
+function clause is_CSR_defined(0x10A) = extensionEnabled(Ext_S) // senvcfg
+
+function clause read_CSR(0x30A) = menvcfg.bits[xlen - 1 .. 0]
+function clause read_CSR(0x31A if xlen == 32) = menvcfg.bits[63 .. 32]
+function clause read_CSR(0x10A) = senvcfg.bits[xlen - 1 .. 0]
+
+function clause write_CSR((0x30A, value) if xlen == 32) = { menvcfg = legalize_menvcfg(menvcfg, menvcfg.bits[63 .. 32] @ value); menvcfg.bits[31 .. 0] }
+function clause write_CSR((0x30A, value) if xlen == 64) = { menvcfg = legalize_menvcfg(menvcfg, value); menvcfg.bits }
+function clause write_CSR((0x31A, value) if xlen == 32) = { menvcfg = legalize_menvcfg(menvcfg, value @ menvcfg.bits[31 .. 0]); menvcfg.bits[63 .. 32] }
+function clause write_CSR(0x10A, value) = { senvcfg = legalize_senvcfg(senvcfg, zero_extend(value)); senvcfg.bits[xlen - 1 .. 0] }
+
+// Return whether or not FIOM is currently active, based on the current
+// privilege and the menvcfg/senvcfg settings. This means that I/O fences
+// imply memory fence.
+function is_fiom_active() -> bool = {
+  match cur_privilege {
+    Machine => false,
+    Supervisor => menvcfg[FIOM] == 0b1,
+    User => (menvcfg[FIOM] | senvcfg[FIOM]) == 0b1,
+  }
+}
+
 /* interrupt processing state */
 
 bitfield Minterrupts : xlenbits = {
@@ -327,8 +415,11 @@ function legalize_mip(o : Minterrupts, v : xlenbits) -> Minterrupts = {
   let v = Mk_Minterrupts(v);
   [o with
     SEI = if extensionEnabled(Ext_S) then v[SEI] else 0b0,
-    STI = if extensionEnabled(Ext_S) then v[STI] else 0b0,
     SSI = if extensionEnabled(Ext_S) then v[SSI] else 0b0,
+    STI = if extensionEnabled(Ext_S) then (
+      // STI is read only if Sstc is enabled and STCE is set (it is equal to stimecmp <= mtime).
+      if extensionEnabled(Ext_Sstc) & menvcfg[STCE] == 0b1 then o[STI] else v[STI]
+    ) else 0b0,
   ]
 }
 
@@ -805,84 +896,6 @@ val get_16_random_bits = impure {
     lem: "plat_get_16_random_bits"
 } : unit -> bits(16)
 
-// envcfg Resisters
-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,
-}
-
-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
-}
-
-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
-}
-
-register menvcfg : MEnvcfg
-register senvcfg : SEnvcfg
-
-mapping clause csr_name_map = 0x30A  <-> "menvcfg"
-mapping clause csr_name_map = 0x31A  <-> "menvcfgh"
-mapping clause csr_name_map = 0x10A  <-> "senvcfg"
-
-function clause is_CSR_defined(0x30A) = extensionEnabled(Ext_U) // menvcfg
-function clause is_CSR_defined(0x31A) = extensionEnabled(Ext_U) & (xlen == 32) // menvcfgh
-function clause is_CSR_defined(0x10A) = extensionEnabled(Ext_S) // senvcfg
-
-function clause read_CSR(0x30A) = menvcfg.bits[xlen - 1 .. 0]
-function clause read_CSR(0x31A if xlen == 32) = menvcfg.bits[63 .. 32]
-function clause read_CSR(0x10A) = senvcfg.bits[xlen - 1 .. 0]
-
-function clause write_CSR((0x30A, value) if xlen == 32) = { menvcfg = legalize_menvcfg(menvcfg, menvcfg.bits[63 .. 32] @ value); menvcfg.bits[31 .. 0] }
-function clause write_CSR((0x30A, value) if xlen == 64) = { menvcfg = legalize_menvcfg(menvcfg, value); menvcfg.bits }
-function clause write_CSR((0x31A, value) if xlen == 32) = { menvcfg = legalize_menvcfg(menvcfg, value @ menvcfg.bits[31 .. 0]); menvcfg.bits[63 .. 32] }
-function clause write_CSR(0x10A, value) = { senvcfg = legalize_senvcfg(senvcfg, zero_extend(value)); senvcfg.bits[xlen - 1 .. 0] }
-
-// Return whether or not FIOM is currently active, based on the current
-// privilege and the menvcfg/senvcfg settings. This means that I/O fences
-// imply memory fence.
-function is_fiom_active() -> bool = {
-  match cur_privilege {
-    Machine => false,
-    Supervisor => menvcfg[FIOM] == 0b1,
-    User => (menvcfg[FIOM] | senvcfg[FIOM]) == 0b1,
-  }
-}
-
 /* vector csrs */
 register vstart : bits(16) /* use the largest possible length of vstart */
 register vl     : xlenbits