Add support for HPM counters

Implements Hardware Performance Monitoring counters, but only by making the registers available. Their values only change if explicitly written to and all values are legal.

All 32 HPM counters are always implemented (the spec says this "should" be the case), and a platform callback `sys_writable_hpm_counters` is added to allow making any of them read-only. I haven't hooked `sys_writable_hpm_counters` up to CLI flags because Codasip does not use the CLI flags and it is quite tedious, and hopefully going to change when riscv-config is used.

Finally this adds `menvcfg` and `senvcfg` to the CSR name map which I forgot to do previously.

c_emulator/riscv_platform.c

@@ -62,6 +62,11 @@ bool sys_enable_writable_misa(unit u)
   return rv_enable_writable_misa;
 }
 
+mach_bits sys_writable_hpm_counters(unit u)
+{
+  return rv_writable_hpm_counters;
+}
+
 bool plat_enable_dirty_update(unit u)
 {
   return rv_enable_dirty_update;

c_emulator/riscv_platform.h

@@ -8,6 +8,7 @@ bool sys_enable_zfinx(unit);
 bool sys_enable_writable_misa(unit);
 bool sys_enable_writable_fiom(unit);
 bool sys_enable_vext(unit);
+mach_bits sys_writable_hpm_counters(unit u);
 
 uint64_t sys_pmp_count(unit);
 uint64_t sys_pmp_grain(unit);

c_emulator/riscv_platform_impl.c

@@ -12,6 +12,7 @@ bool rv_enable_next = false;
 bool rv_enable_writable_misa = true;
 bool rv_enable_fdext = true;
 bool rv_enable_vext = true;
+uint64_t rv_writable_hpm_counters = 0xFFFFFFFF;
 
 bool rv_enable_dirty_update = false;
 bool rv_enable_misaligned = false;

c_emulator/riscv_platform_impl.h

@@ -16,6 +16,7 @@ extern bool rv_enable_rvc;
 extern bool rv_enable_next;
 extern bool rv_enable_fdext;
 extern bool rv_enable_vext;
+extern uint64_t rv_writable_hpm_counters;
 extern bool rv_enable_writable_misa;
 extern bool rv_enable_dirty_update;
 extern bool rv_enable_misaligned;

model/prelude.sail

@@ -178,22 +178,31 @@ val to_bits : forall 'l, 'l >= 0.(atom('l), int) -> bits('l)
 function to_bits (l, n) = get_slice_int(l, n, 0)
 
 infix 4 <_s
+infix 4 >_s
+infix 4 <=_s
 infix 4 >=_s
 infix 4 <_u
-infix 4 >=_u
+infix 4 >_u
 infix 4 <=_u
+infix 4 >=_u
 
 val operator <_s  : forall 'n, 'n > 0. (bits('n), bits('n)) -> bool
+val operator >_s  : forall 'n, 'n > 0. (bits('n), bits('n)) -> bool
+val operator <=_s : forall 'n, 'n > 0. (bits('n), bits('n)) -> bool
 val operator >=_s : forall 'n, 'n > 0. (bits('n), bits('n)) -> bool
 val operator <_u  : forall 'n. (bits('n), bits('n)) -> bool
-val operator >=_u : forall 'n. (bits('n), bits('n)) -> bool
+val operator >_u  : forall 'n. (bits('n), bits('n)) -> bool
 val operator <=_u : forall 'n. (bits('n), bits('n)) -> bool
+val operator >=_u : forall 'n. (bits('n), bits('n)) -> bool
 
 function operator <_s  (x, y) = signed(x) < signed(y)
+function operator >_s  (x, y) = signed(x) > signed(y)
+function operator <=_s (x, y) = signed(x) <= signed(y)
 function operator >=_s (x, y) = signed(x) >= signed(y)
 function operator <_u  (x, y) = unsigned(x) < unsigned(y)
-function operator >=_u (x, y) = unsigned(x) >= unsigned(y)
+function operator >_u  (x, y) = unsigned(x) > unsigned(y)
 function operator <=_u (x, y) = unsigned(x) <= unsigned(y)
+function operator >=_u (x, y) = unsigned(x) >= unsigned(y)
 
 infix 7 >>
 infix 7 <<

model/riscv_csr_map.sail

@@ -94,10 +94,69 @@ mapping clause csr_name_map = 0x015  <-> "seed"
 mapping clause csr_name_map = 0xC00  <-> "cycle"
 mapping clause csr_name_map = 0xC01  <-> "time"
 mapping clause csr_name_map = 0xC02  <-> "instret"
+mapping clause csr_name_map = 0xC03  <-> "hpmcounter3"
+mapping clause csr_name_map = 0xC04  <-> "hpmcounter4"
+mapping clause csr_name_map = 0xC05  <-> "hpmcounter5"
+mapping clause csr_name_map = 0xC06  <-> "hpmcounter6"
+mapping clause csr_name_map = 0xC07  <-> "hpmcounter7"
+mapping clause csr_name_map = 0xC08  <-> "hpmcounter8"
+mapping clause csr_name_map = 0xC09  <-> "hpmcounter9"
+mapping clause csr_name_map = 0xC0A  <-> "hpmcounter10"
+mapping clause csr_name_map = 0xC0B  <-> "hpmcounter11"
+mapping clause csr_name_map = 0xC0C  <-> "hpmcounter12"
+mapping clause csr_name_map = 0xC0D  <-> "hpmcounter13"
+mapping clause csr_name_map = 0xC0E  <-> "hpmcounter14"
+mapping clause csr_name_map = 0xC0F  <-> "hpmcounter15"
+mapping clause csr_name_map = 0xC10  <-> "hpmcounter16"
+mapping clause csr_name_map = 0xC11  <-> "hpmcounter17"
+mapping clause csr_name_map = 0xC12  <-> "hpmcounter18"
+mapping clause csr_name_map = 0xC13  <-> "hpmcounter19"
+mapping clause csr_name_map = 0xC14  <-> "hpmcounter20"
+mapping clause csr_name_map = 0xC15  <-> "hpmcounter21"
+mapping clause csr_name_map = 0xC16  <-> "hpmcounter22"
+mapping clause csr_name_map = 0xC17  <-> "hpmcounter23"
+mapping clause csr_name_map = 0xC18  <-> "hpmcounter24"
+mapping clause csr_name_map = 0xC19  <-> "hpmcounter25"
+mapping clause csr_name_map = 0xC1A  <-> "hpmcounter26"
+mapping clause csr_name_map = 0xC1B  <-> "hpmcounter27"
+mapping clause csr_name_map = 0xC1C  <-> "hpmcounter28"
+mapping clause csr_name_map = 0xC1D  <-> "hpmcounter29"
+mapping clause csr_name_map = 0xC1E  <-> "hpmcounter30"
+mapping clause csr_name_map = 0xC1F  <-> "hpmcounter31"
+
 mapping clause csr_name_map = 0xC80  <-> "cycleh"
 mapping clause csr_name_map = 0xC81  <-> "timeh"
 mapping clause csr_name_map = 0xC82  <-> "instreth"
-/* TODO: other hpm counters */
+mapping clause csr_name_map = 0xC83  <-> "hpmcounter3h"
+mapping clause csr_name_map = 0xC84  <-> "hpmcounter4h"
+mapping clause csr_name_map = 0xC85  <-> "hpmcounter5h"
+mapping clause csr_name_map = 0xC86  <-> "hpmcounter6h"
+mapping clause csr_name_map = 0xC87  <-> "hpmcounter7h"
+mapping clause csr_name_map = 0xC88  <-> "hpmcounter8h"
+mapping clause csr_name_map = 0xC89  <-> "hpmcounter9h"
+mapping clause csr_name_map = 0xC8A  <-> "hpmcounter10h"
+mapping clause csr_name_map = 0xC8B  <-> "hpmcounter11h"
+mapping clause csr_name_map = 0xC8C  <-> "hpmcounter12h"
+mapping clause csr_name_map = 0xC8D  <-> "hpmcounter13h"
+mapping clause csr_name_map = 0xC8E  <-> "hpmcounter14h"
+mapping clause csr_name_map = 0xC8F  <-> "hpmcounter15h"
+mapping clause csr_name_map = 0xC90  <-> "hpmcounter16h"
+mapping clause csr_name_map = 0xC91  <-> "hpmcounter17h"
+mapping clause csr_name_map = 0xC92  <-> "hpmcounter18h"
+mapping clause csr_name_map = 0xC93  <-> "hpmcounter19h"
+mapping clause csr_name_map = 0xC94  <-> "hpmcounter20h"
+mapping clause csr_name_map = 0xC95  <-> "hpmcounter21h"
+mapping clause csr_name_map = 0xC96  <-> "hpmcounter22h"
+mapping clause csr_name_map = 0xC97  <-> "hpmcounter23h"
+mapping clause csr_name_map = 0xC98  <-> "hpmcounter24h"
+mapping clause csr_name_map = 0xC99  <-> "hpmcounter25h"
+mapping clause csr_name_map = 0xC9A  <-> "hpmcounter26h"
+mapping clause csr_name_map = 0xC9B  <-> "hpmcounter27h"
+mapping clause csr_name_map = 0xC9C  <-> "hpmcounter28h"
+mapping clause csr_name_map = 0xC9D  <-> "hpmcounter29h"
+mapping clause csr_name_map = 0xC9E  <-> "hpmcounter30h"
+mapping clause csr_name_map = 0xC9F  <-> "hpmcounter31h"
+
 /* supervisor trap setup */
 mapping clause csr_name_map = 0x100  <-> "sstatus"
 mapping clause csr_name_map = 0x102  <-> "sedeleg"
@@ -127,6 +186,35 @@ mapping clause csr_name_map = 0x304  <-> "mie"
 mapping clause csr_name_map = 0x305  <-> "mtvec"
 mapping clause csr_name_map = 0x306  <-> "mcounteren"
 mapping clause csr_name_map = 0x320  <-> "mcountinhibit"
+mapping clause csr_name_map = 0x323  <-> "mhpmevent3"
+mapping clause csr_name_map = 0x324  <-> "mhpmevent4"
+mapping clause csr_name_map = 0x325  <-> "mhpmevent5"
+mapping clause csr_name_map = 0x326  <-> "mhpmevent6"
+mapping clause csr_name_map = 0x327  <-> "mhpmevent7"
+mapping clause csr_name_map = 0x328  <-> "mhpmevent8"
+mapping clause csr_name_map = 0x329  <-> "mhpmevent9"
+mapping clause csr_name_map = 0x32A  <-> "mhpmevent10"
+mapping clause csr_name_map = 0x32B  <-> "mhpmevent11"
+mapping clause csr_name_map = 0x32C  <-> "mhpmevent12"
+mapping clause csr_name_map = 0x32D  <-> "mhpmevent13"
+mapping clause csr_name_map = 0x32E  <-> "mhpmevent14"
+mapping clause csr_name_map = 0x32F  <-> "mhpmevent15"
+mapping clause csr_name_map = 0x330  <-> "mhpmevent16"
+mapping clause csr_name_map = 0x331  <-> "mhpmevent17"
+mapping clause csr_name_map = 0x332  <-> "mhpmevent18"
+mapping clause csr_name_map = 0x333  <-> "mhpmevent19"
+mapping clause csr_name_map = 0x334  <-> "mhpmevent20"
+mapping clause csr_name_map = 0x335  <-> "mhpmevent21"
+mapping clause csr_name_map = 0x336  <-> "mhpmevent22"
+mapping clause csr_name_map = 0x337  <-> "mhpmevent23"
+mapping clause csr_name_map = 0x338  <-> "mhpmevent24"
+mapping clause csr_name_map = 0x339  <-> "mhpmevent25"
+mapping clause csr_name_map = 0x33A  <-> "mhpmevent26"
+mapping clause csr_name_map = 0x33B  <-> "mhpmevent27"
+mapping clause csr_name_map = 0x33C  <-> "mhpmevent28"
+mapping clause csr_name_map = 0x33D  <-> "mhpmevent29"
+mapping clause csr_name_map = 0x33E  <-> "mhpmevent30"
+mapping clause csr_name_map = 0x33F  <-> "mhpmevent31"
 /* machine trap handling */
 mapping clause csr_name_map = 0x340  <-> "mscratch"
 mapping clause csr_name_map = 0x341  <-> "mepc"
@@ -217,9 +305,66 @@ mapping clause csr_name_map = 0x3EF  <-> "pmpaddr63"
 /* machine counters/timers */
 mapping clause csr_name_map = 0xB00  <-> "mcycle"
 mapping clause csr_name_map = 0xB02  <-> "minstret"
+mapping clause csr_name_map = 0xB03  <-> "mhpmcounter3"
+mapping clause csr_name_map = 0xB04  <-> "mhpmcounter4"
+mapping clause csr_name_map = 0xB05  <-> "mhpmcounter5"
+mapping clause csr_name_map = 0xB06  <-> "mhpmcounter6"
+mapping clause csr_name_map = 0xB07  <-> "mhpmcounter7"
+mapping clause csr_name_map = 0xB08  <-> "mhpmcounter8"
+mapping clause csr_name_map = 0xB09  <-> "mhpmcounter9"
+mapping clause csr_name_map = 0xB0A  <-> "mhpmcounter10"
+mapping clause csr_name_map = 0xB0B  <-> "mhpmcounter11"
+mapping clause csr_name_map = 0xB0C  <-> "mhpmcounter12"
+mapping clause csr_name_map = 0xB0D  <-> "mhpmcounter13"
+mapping clause csr_name_map = 0xB0E  <-> "mhpmcounter14"
+mapping clause csr_name_map = 0xB0F  <-> "mhpmcounter15"
+mapping clause csr_name_map = 0xB10  <-> "mhpmcounter16"
+mapping clause csr_name_map = 0xB11  <-> "mhpmcounter17"
+mapping clause csr_name_map = 0xB12  <-> "mhpmcounter18"
+mapping clause csr_name_map = 0xB13  <-> "mhpmcounter19"
+mapping clause csr_name_map = 0xB14  <-> "mhpmcounter20"
+mapping clause csr_name_map = 0xB15  <-> "mhpmcounter21"
+mapping clause csr_name_map = 0xB16  <-> "mhpmcounter22"
+mapping clause csr_name_map = 0xB17  <-> "mhpmcounter23"
+mapping clause csr_name_map = 0xB18  <-> "mhpmcounter24"
+mapping clause csr_name_map = 0xB19  <-> "mhpmcounter25"
+mapping clause csr_name_map = 0xB1A  <-> "mhpmcounter26"
+mapping clause csr_name_map = 0xB1B  <-> "mhpmcounter27"
+mapping clause csr_name_map = 0xB1C  <-> "mhpmcounter28"
+mapping clause csr_name_map = 0xB1D  <-> "mhpmcounter29"
+mapping clause csr_name_map = 0xB1E  <-> "mhpmcounter30"
+mapping clause csr_name_map = 0xB1F  <-> "mhpmcounter31"
 mapping clause csr_name_map = 0xB80  <-> "mcycleh"
 mapping clause csr_name_map = 0xB82  <-> "minstreth"
-/* TODO: other hpm counters and events */
+mapping clause csr_name_map = 0xB83  <-> "mhpmcounter3h"
+mapping clause csr_name_map = 0xB84  <-> "mhpmcounter4h"
+mapping clause csr_name_map = 0xB85  <-> "mhpmcounter5h"
+mapping clause csr_name_map = 0xB86  <-> "mhpmcounter6h"
+mapping clause csr_name_map = 0xB87  <-> "mhpmcounter7h"
+mapping clause csr_name_map = 0xB88  <-> "mhpmcounter8h"
+mapping clause csr_name_map = 0xB89  <-> "mhpmcounter9h"
+mapping clause csr_name_map = 0xB8A  <-> "mhpmcounter10h"
+mapping clause csr_name_map = 0xB8B  <-> "mhpmcounter11h"
+mapping clause csr_name_map = 0xB8C  <-> "mhpmcounter12h"
+mapping clause csr_name_map = 0xB8D  <-> "mhpmcounter13h"
+mapping clause csr_name_map = 0xB8E  <-> "mhpmcounter14h"
+mapping clause csr_name_map = 0xB8F  <-> "mhpmcounter15h"
+mapping clause csr_name_map = 0xB90  <-> "mhpmcounter16h"
+mapping clause csr_name_map = 0xB91  <-> "mhpmcounter17h"
+mapping clause csr_name_map = 0xB92  <-> "mhpmcounter18h"
+mapping clause csr_name_map = 0xB93  <-> "mhpmcounter19h"
+mapping clause csr_name_map = 0xB94  <-> "mhpmcounter20h"
+mapping clause csr_name_map = 0xB95  <-> "mhpmcounter21h"
+mapping clause csr_name_map = 0xB96  <-> "mhpmcounter22h"
+mapping clause csr_name_map = 0xB97  <-> "mhpmcounter23h"
+mapping clause csr_name_map = 0xB98  <-> "mhpmcounter24h"
+mapping clause csr_name_map = 0xB99  <-> "mhpmcounter25h"
+mapping clause csr_name_map = 0xB9A  <-> "mhpmcounter26h"
+mapping clause csr_name_map = 0xB9B  <-> "mhpmcounter27h"
+mapping clause csr_name_map = 0xB9C  <-> "mhpmcounter28h"
+mapping clause csr_name_map = 0xB9D  <-> "mhpmcounter29h"
+mapping clause csr_name_map = 0xB9E  <-> "mhpmcounter30h"
+mapping clause csr_name_map = 0xB9F  <-> "mhpmcounter31h"
 /* trigger/debug */
 mapping clause csr_name_map = 0x7a0  <-> "tselect"
 mapping clause csr_name_map = 0x7a1  <-> "tdata1"

model/riscv_insts_zicsr.sail

@@ -103,6 +103,9 @@ function readCSR csr : csreg -> xlenbits = {
     (0x31A, 32) => menvcfg.bits[63 .. 32],
     (0x320,  _) => zero_extend(mcountinhibit.bits),
 
+    /* Hardware Performance Monitoring event selection */
+    (0b0011001 /* 0x320 */ @ index : bits(5), _) if index >_u 0b00010 => read_mhpmevent(hpmidx_from_bits(index)),
+
     (0x340,  _) => mscratch,
     (0x341,  _) => get_xret_target(Machine) & pc_alignment_mask(),
     (0x342,  _) => mcause.bits,
@@ -123,6 +126,10 @@ function readCSR csr : csreg -> xlenbits = {
     (0xB80, 32) => mcycle[63 .. 32],
     (0xB82, 32) => minstret[63 .. 32],
 
+    /* Hardware Performance Monitoring machine mode counters */
+    (0b1011000 /* 0xB00 */ @ index : bits(5), _) if index >_u 0b00010 => read_mhpmcounter(hpmidx_from_bits(index)),
+    (0b1011100 /* 0xB80 */ @ index : bits(5), 32) if index >_u 0b00010 => read_mhpmcounterh(hpmidx_from_bits(index)),
+
     /* vector */
     (0x008, _) => zero_extend(vstart),
     (0x009, _) => zero_extend(vxsat),
@@ -158,6 +165,10 @@ function readCSR csr : csreg -> xlenbits = {
     (0xC81, 32) => mtime[63 .. 32],
     (0xC82, 32) => minstret[63 .. 32],
 
+    /* Hardware Performance Monitoring user mode counters */
+    (0b1100000 /* 0xC00 */ @ index : bits(5), _) if index >_u 0b00010 => read_mhpmcounter(hpmidx_from_bits(index)),
+    (0b1100100 /* 0xC80 */ @ index : bits(5), 32) if index >_u 0b00010 => read_mhpmcounterh(hpmidx_from_bits(index)),
+
     /* user mode: Zkr */
     (0x015,  _) => read_seed_csr(),
 
@@ -213,6 +224,23 @@ function writeCSR (csr : csreg, value : xlenbits) -> unit = {
     (0xB80, 32) => { mcycle[63 .. 32] = value; Some(value) },
     (0xB82, 32) => { minstret[63 .. 32] = value; minstret_increment = false; Some(value) },
 
+    /* Hardware Performance Monitoring machine mode counters */
+    (0b0011001 /* 0x320 */ @ index : bits(5), _) if index >_u 0b00010 => {
+      let index = hpmidx_from_bits(index);
+      write_mhpmevent(index, value);
+      Some(read_mhpmevent(index))
+    },
+    (0b1011000 /* 0xB00 */ @ index : bits(5), _) if index >_u 0b00010 => {
+      let index = hpmidx_from_bits(index);
+      write_mhpmcounter(index, value);
+      Some(read_mhpmcounter(index))
+    },
+    (0b1011100 /* 0xB80 */ @ index : bits(5), 32) if index >_u 0b00010 => {
+      let index = hpmidx_from_bits(index);
+      write_mhpmcounterh(index, value);
+      Some(read_mhpmcounterh(index))
+    },
+
     /* trigger/debug */
     (0x7a0,  _) => { tselect = value; Some(tselect) },
 

model/riscv_sys_control.sail

@@ -110,12 +110,20 @@ function is_CSR_defined (csr : csreg, p : Privilege) -> bool =
     0x3C @ idx : bits(4) => p == Machine & sys_pmp_count() > unsigned(0b01 @ idx),
     0x3D @ idx : bits(4) => p == Machine & sys_pmp_count() > unsigned(0b10 @ idx),
     0x3E @ idx : bits(4) => p == Machine & sys_pmp_count() > unsigned(0b11 @ idx),
+
+    /* counters */
+    0b0011001 /* 0x320 */ @ index : bits(5) if index >_u 0b00010 => haveZihpm() & p == Machine, // mhpmevent3..31
+
     0xB00 => p == Machine, // mcycle
     0xB02 => p == Machine, // minstret
 
+    0b1011000 /* 0xB00 */ @ index : bits(5) if index >_u 0b00010 => haveZihpm() & p == Machine, // mhpmcounter3..31
+
     0xB80 => p == Machine & (sizeof(xlen) == 32), // mcycleh
     0xB82 => p == Machine & (sizeof(xlen) == 32), // minstreth
 
+    0b1011100 /* 0xB80 */ @ index : bits(5) if index >_u 0b00010 => haveZihpm() & p == Machine & (sizeof(xlen) == 32), // mhpmcounterh3..31
+
     /* disabled trigger/debug module */
     0x7a0 => p == Machine,
 
@@ -143,10 +151,14 @@ function is_CSR_defined (csr : csreg, p : Privilege) -> bool =
     0xC01 => haveUsrMode(),    // time
     0xC02 => haveUsrMode(),    // instret
 
+    0b1100000 /* 0xC00 */ @ index : bits(5) if index >_u 0b00010 => haveZihpm() & haveUsrMode(), // mhpmcounter3..31
+
     0xC80 => haveUsrMode() & (sizeof(xlen) == 32),     // cycleh
     0xC81 => haveUsrMode() & (sizeof(xlen) == 32),     // timeh
     0xC82 => haveUsrMode() & (sizeof(xlen) == 32),     // instreth
 
+    0b1100100 /* 0xC80 */ @ index : bits(5) if index >_u 0b00010 => haveZihpm() & haveUsrMode() & (sizeof(xlen) == 32), // mhpmcounterh3..31
+
     /* user mode: Zkr */
     0x015 => haveZkr(),
 
@@ -162,22 +174,24 @@ function check_CSR_access(csrrw, csrpr, p, isWrite) =
 function check_TVM_SATP(csr : csreg, p : Privilege) -> bool =
   not(csr == 0x180 & p == Supervisor & mstatus[TVM] == 0b1)
 
-function check_Counteren(csr : csreg, p : Privilege) -> bool =
-  match(csr, p) {
-    (0xC00, Supervisor) => mcounteren[CY] == 0b1,
-    (0xC01, Supervisor) => mcounteren[TM] == 0b1,
-    (0xC02, Supervisor) => mcounteren[IR] == 0b1,
+// There are several features that are controlled by machine/supervisor enable
+// bits (m/senvcfg, m/scounteren, etc.). This abstracts that logic.
+function mode_bit_enabled(p : Privilege, machine_bit : bit, supervisor_bit : bit) -> bool = match p {
+  Machine => true,
+  Supervisor => machine_bit == bitone,
+  User => machine_bit == bitone & (not(haveSupMode()) | supervisor_bit == bitone),
+}
 
-    (0xC00, User) => mcounteren[CY] == 0b1 & (not(haveSupMode()) | scounteren[CY] == 0b1),
-    (0xC01, User) => mcounteren[TM] == 0b1 & (not(haveSupMode()) | scounteren[TM] == 0b1),
-    (0xC02, User) => mcounteren[IR] == 0b1 & (not(haveSupMode()) | scounteren[IR] == 0b1),
-
-    (_, _) => /* no HPM counters for now */
-              if   0xC03 <=_u csr & csr <=_u 0xC1F
-              then false
-              else true
-  }
+// 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 | csr >_u 0xC1F
+  then return true;
 
+  // Check the relevant bit in m/scounteren.
+  let index = unsigned(csr[4 .. 0]);
+  mode_bit_enabled(p, mcounteren.bits()[index], scounteren.bits()[index])
+}
 
 /* Seed may only be accessed if we are doing a write, and access has been
  * allowed in the current priv mode