AIA: Add inaccessible vstopei CSR

AIA introduces the concept of inaccessible CSR, where accessing from
M-mode or HS-mode raises an illegal instruction exception, but doing so
from VS-mode or VU-mode raises a virtual instruction exception.

Without IMSIC, mtopei and stopei do not exist. In contrast, vstopei is
an inaccessible CSR even without IMSIC, i.e., exits and is HS-qualified.

In summary, accessing stopei from M-mode or HS-mode (v=0) raises illegal
instruction, and accessing stopei (actually vstopei) from VS-mode or
VU-mode (v=1) raises virtual instruction.

riscv/csr_init.cc

@@ -527,6 +527,7 @@ void state_t::csr_init(processor_t* const proc, reg_t max_isa)
 
   if (proc->extension_enabled_const(EXT_SSAIA)) { // Included by EXT_SMAIA
     add_supervisor_csr(CSR_STOPI, std::make_shared<nonvirtual_stopi_csr_t>(proc, CSR_STOPI));
+    add_supervisor_csr(CSR_STOPEI, std::make_shared<inaccessible_csr_t>(proc, CSR_STOPEI));
     auto hvien = std::make_shared<const_csr_t>(proc, CSR_HVIEN, 0);
     auto hviprio1 = std::make_shared<const_csr_t>(proc, CSR_HVIPRIO1, 0);
     auto hviprio2 = std::make_shared<const_csr_t>(proc, CSR_HVIPRIO2, 0);

riscv/csrs.cc

@@ -2080,3 +2080,14 @@ reg_t nonvirtual_stopi_csr_t::read() const noexcept {
 bool nonvirtual_stopi_csr_t::unlogged_write(const reg_t UNUSED val) noexcept {
   return false;
 }
+
+inaccessible_csr_t::inaccessible_csr_t(processor_t* const proc, const reg_t addr):
+  csr_t(proc, addr) {
+}
+
+void inaccessible_csr_t::verify_permissions(insn_t insn, bool write) const {
+  if (state->v)
+    throw trap_virtual_instruction(insn.bits());
+  else
+    throw trap_illegal_instruction(insn.bits());
+}

riscv/csrs.h

@@ -957,4 +957,13 @@ class nonvirtual_stopi_csr_t: public csr_t {
  protected:
   bool unlogged_write(const reg_t val) noexcept override;
 };
+
+class inaccessible_csr_t: public csr_t {
+ public:
+  inaccessible_csr_t(processor_t* const proc, const reg_t addr);
+  virtual void verify_permissions(insn_t insn, bool write) const override;
+  reg_t read() const noexcept override { return 0; }
+ protected:
+  bool unlogged_write(const reg_t val) noexcept override { return false; }
+};
 #endif