riscv64,system,priv: read clint for rdtime

* rdtime may be located in the middle of a basic block. Raising an
  illegal instruction exception will also flush the tcache, since we can
  not maintain the continuity of tcache entry. This will reduce
  performance of such rdtime instruciton is executed frequently.
* To fix this, we let rdtime return the right value instead of raising
  an exception.

src/isa/riscv64/clint.c

@@ -21,6 +21,11 @@ void update_clint() {
   mip->mtip = (clint_base[CLINT_MTIME] >= clint_base[CLINT_MTIMECMP]);
 }
 
+uint64_t clint_uptime() {
+  update_clint();
+  return clint_base[CLINT_MTIME];
+}
+
 static void clint_io_handler(uint32_t offset, int len, bool is_write) {
   update_clint();
 }

src/isa/riscv64/local-include/csr.h

@@ -16,7 +16,8 @@
   f(sscratch   , 0x140) f(sepc       , 0x141) f(scause     , 0x142) \
   f(stval      , 0x143) f(sip        , 0x144) \
   f(satp       , 0x180) \
-  f(fflags     , 0x001) f(frm        , 0x002) f(fcsr       , 0x003)
+  f(fflags     , 0x001) f(frm        , 0x002) f(fcsr       , 0x003) \
+  f(mtime      , 0xc01)
 
 #define CSR_STRUCT_START(name) \
   typedef union { \
@@ -220,6 +221,9 @@ CSR_STRUCT_START(fcsr)
   };
 CSR_STRUCT_END(fcsr)
 
+CSR_STRUCT_START(mtime)
+CSR_STRUCT_END(mtime)
+
 #define CSRS_DECL(name, addr) extern concat(name, _t)* const name;
 MAP(CSRS, CSRS_DECL)
 

src/isa/riscv64/system/priv.c

@@ -1,9 +1,9 @@
 #include "../local-include/csr.h"
 #include "../local-include/rtl.h"
-#include "../local-include/intr.h"
 #include <cpu/cpu.h>
 
 int update_mmu_state();
+uint64_t clint_uptime();
 
 static word_t csr_array[4096] = {};
 
@@ -39,6 +39,7 @@ static inline word_t csr_read(word_t *src) {
   else if (is_read(fcsr))   { return fcsr->val & FCSR_MASK; }
   else if (is_read(fflags)) { return fcsr->fflags.val; }
   else if (is_read(frm))    { return fcsr->frm; }
+  else if (is_read(mtime))  { return clint_uptime(); }
   return *src;
 }
 
@@ -76,7 +77,6 @@ word_t csrid_read(uint32_t csrid) {
 }
 
 static void csrrw(rtlreg_t *dest, const rtlreg_t *src, uint32_t csrid) {
-  if (csrid == 0xc01) { longjmp_exception(EX_II); } // time
   word_t *csr = csr_decode(csrid);
   word_t tmp = (src != NULL ? *src : 0);
   if (dest != NULL) { *dest = csr_read(csr); }