AIA: Let sie[n] be writable when mideleg[n]=0 and mvien[n]=1

The AIA spec specifies the sie[n] behavior conditionally:
(1) When mideleg[n]=0 and mvien[n]=0, sie[n] is read-only 0.
(2) When mideleg[n]=0 and mvien[n]=1, sie[n] is writable.
(3) When mideleg[n]=1, sie[n] is an alias of mie[n].

Points (1) and (3) describe the same behavior without AIA. This commit
provides the behavior of point (2).

riscv/csr_init.cc

@@ -150,7 +150,7 @@ void state_t::csr_init(processor_t* const proc, reg_t max_isa)
   add_hypervisor_csr(CSR_HIP, std::make_shared<mip_proxy_csr_t>(proc, CSR_HIP, hip_hie_accr));
   add_hypervisor_csr(CSR_HVIP, hvip = std::make_shared<hvip_csr_t>(proc, CSR_HVIP, 0));
 
-  auto nonvirtual_sie = std::make_shared<mie_proxy_csr_t>(proc, CSR_SIE, sip_sie_accr);
+  auto nonvirtual_sie = std::make_shared<sie_csr_t>(proc, CSR_SIE, sip_sie_accr);
   auto vsie = std::make_shared<mie_proxy_csr_t>(proc, CSR_VSIE, vsip_vsie_accr);
   add_hypervisor_csr(CSR_VSIE, vsie);
   add_supervisor_csr(CSR_SIE, std::make_shared<virtualized_csr_t>(proc, nonvirtual_sie, vsie));

riscv/csrs.cc

@@ -979,6 +979,23 @@ bool sip_csr_t::unlogged_write(const reg_t val) noexcept {
   return mip_proxy_csr_t::unlogged_write(val & ~mask);
 }
 
+sie_csr_t::sie_csr_t(processor_t* const proc, const reg_t addr, generic_int_accessor_t_p accr):
+  mie_proxy_csr_t(proc, addr, accr),
+  val(0) {
+}
+
+reg_t sie_csr_t::read() const noexcept {
+  const reg_t mask = ~state->mideleg->read() & state->mvien->read();
+  return (mie_proxy_csr_t::read() & ~mask) | (val & mask);
+}
+
+bool sie_csr_t::unlogged_write(const reg_t val) noexcept {
+  const reg_t mask = ~state->mideleg->read() & state->mvien->read();
+  this->val = (this->val & ~mask) | (val & mask);
+  mie_proxy_csr_t::unlogged_write(val & ~mask);
+  return true;
+}
+
 // implement class masked_csr_t
 masked_csr_t::masked_csr_t(processor_t* const proc, const reg_t addr, const reg_t mask, const reg_t init):
   basic_csr_t(proc, addr, init),

riscv/csrs.h

@@ -470,6 +470,16 @@ class sip_csr_t: public mip_proxy_csr_t {
   virtual bool unlogged_write(const reg_t val) noexcept override;
 };
 
+class sie_csr_t: public mie_proxy_csr_t {
+ public:
+  sie_csr_t(processor_t* const proc, const reg_t addr, generic_int_accessor_t_p accr);
+  virtual reg_t read() const noexcept override;
+ protected:
+  virtual bool unlogged_write(const reg_t val) noexcept override;
+ private:
+  reg_t val;
+};
+
 // For CSRs with certain bits hardwired
 class masked_csr_t: public basic_csr_t {
  public: