refactor: add custom CSR class, mevent_csr_t

The masked_csr_t does not meet the behavior of mevent
because the misa.H is not read-only (hardwired).
(fix in the next commit)

h/t @kwalker27
reported at https://github.com/riscv-software-src/riscv-isa-sim/pull/1154

riscv/csrs.cc

@@ -1054,6 +1054,18 @@ void counter_proxy_csr_t::verify_permissions(insn_t insn, bool write) const {
   }
 }
 
+mevent_csr_t::mevent_csr_t(processor_t* const proc, const reg_t addr):
+  basic_csr_t(proc, addr, 0) {
+}
+
+bool mevent_csr_t::unlogged_write(const reg_t val) noexcept {
+  const reg_t mask = proc->extension_enabled(EXT_SSCOFPMF) ? MHPMEVENT_OF | MHPMEVENT_MINH
+    | (proc->extension_enabled_const('U') ? MHPMEVENT_UINH : 0)
+    | (proc->extension_enabled_const('S') ? MHPMEVENT_SINH : 0)
+    | (proc->extension_enabled_const('H') ? MHPMEVENT_VUINH | MHPMEVENT_VSINH : 0) : 0;
+  return basic_csr_t::unlogged_write((read() & ~mask) | (val & mask));
+}
+
 hypervisor_csr_t::hypervisor_csr_t(processor_t* const proc, const reg_t addr):
   basic_csr_t(proc, addr, 0) {
 }

riscv/csrs.h

@@ -559,6 +559,13 @@ class counter_proxy_csr_t: public proxy_csr_t {
   bool myenable(csr_t_p counteren) const noexcept;
 };
 
+class mevent_csr_t: public basic_csr_t {
+ public:
+  mevent_csr_t(processor_t* const proc, const reg_t addr);
+ protected:
+  virtual bool unlogged_write(const reg_t val) noexcept override;
+};
+
 // For machine-level CSRs that only exist with Hypervisor
 class hypervisor_csr_t: public basic_csr_t {
  public:

riscv/processor.cc

@@ -238,11 +238,7 @@ void state_t::reset(processor_t* const proc, reg_t max_isa)
     const reg_t which_mcounterh = CSR_MHPMCOUNTER3H + i - 3;
     const reg_t which_counter = CSR_HPMCOUNTER3 + i - 3;
     const reg_t which_counterh = CSR_HPMCOUNTER3H + i - 3;
-    const reg_t mevent_mask = proc->extension_enabled_const(EXT_SSCOFPMF) ? MHPMEVENT_OF | MHPMEVENT_MINH
-      | (proc->extension_enabled_const('U') ? MHPMEVENT_UINH : 0)
-      | (proc->extension_enabled_const('S') ? MHPMEVENT_SINH : 0)
-      | (proc->extension_enabled_const('H') ? MHPMEVENT_VUINH | MHPMEVENT_VSINH : 0) : 0;
-    mevent[i - 3] = std::make_shared<masked_csr_t>(proc, which_mevent, mevent_mask, 0);
+    mevent[i - 3] = std::make_shared<mevent_csr_t>(proc, which_mevent);
     auto mcounter = std::make_shared<const_csr_t>(proc, which_mcounter, 0);
     csrmap[which_mcounter] = mcounter;