Can jump to and execute Debug ROM.

Connect with gdb, and the core will jump to Debug ROM and start
executing it. Then it crashes when it jumps to 0x400 because Debug RAM
isn't implemented (and doesn't live there anyway, for now).

debug_rom/debug_rom.c → riscv/debug_rom.h

@@ -1,4 +1,4 @@
-unsigned char debug_rom_raw[] = {
+static unsigned char debug_rom_raw[] = {
   0x6f, 0x00, 0x40, 0x05, 0xf3, 0x24, 0x00, 0xf1, 0x23, 0x24, 0x90, 0x10,
   0xf3, 0x24, 0x00, 0x79, 0x93, 0xf4, 0x04, 0x40, 0x63, 0x94, 0x04, 0x08,
   0xf3, 0x24, 0x00, 0xf0, 0x63, 0xc6, 0x04, 0x00, 0x83, 0x24, 0xc0, 0x43,
@@ -15,4 +15,4 @@ unsigned char debug_rom_raw[] = {
   0x73, 0x24, 0x00, 0x79, 0x13, 0x74, 0x04, 0x40, 0xe3, 0x0c, 0x04, 0xfe,
   0x6f, 0xf0, 0x1f, 0xfc
 };
-unsigned int debug_rom_raw_len = 172;
+static unsigned int debug_rom_raw_len = 172;

debug_rom/Makefile → riscv/debug_rom/Makefile

@@ -7,7 +7,7 @@ OBJCOPY = $(RISCV)/bin/riscv64-unknown-elf-objcopy
 %.o:	%.S
 	$(CC) -c $<
 
-debug_rom.c:	debug_rom.raw
+debug_rom.cc:	debug_rom.raw
 	xxd -i $^ > $@
 
 debug_rom.raw:	debug_rom

riscv/debug_rom/debug_rom.h

@@ -0,0 +1,2 @@
+extern unsigned char *debug_rom_raw;
+extern unsigned int debug_rom_raw_len;

riscv/decode.h

@@ -14,6 +14,7 @@
 #include "config.h"
 #include "common.h"
 #include <cinttypes>
+#include "debug_rom.h"
 
 typedef int64_t sreg_t;
 typedef uint64_t reg_t;
@@ -236,7 +237,9 @@ private:
 #define DCSR_CAUSE_STEPPED      4
 #define DCSR_CAUSE_HALT         5
 
-#define DEBUG_RAM               0xfffffc00       // TODO: 0x400
-#define DEBUG_ROM_ENTRY         0xfffff800       // TODO: 0x800
+#define DEBUG_RAM_START         0xfffffffffffffc00       // TODO: 0x400
+#define DEBUG_RAM_END           (DEBUG_RAM_START + 64)
+#define DEBUG_ROM_START         0xfffffffffffff800       // TODO: 0x800
+#define DEBUG_ROM_END           (DEBUG_ROM_START + debug_rom_raw_len)
 
 #endif

riscv/mmu.cc

@@ -54,6 +54,7 @@ reg_t mmu_t::translate(reg_t addr, access_type type)
 const uint16_t* mmu_t::fetch_slow_path(reg_t addr)
 {
   reg_t paddr = translate(addr, FETCH);
+
   if (sim->addr_is_mem(paddr)) {
     refill_tlb(addr, paddr, FETCH);
     return (const uint16_t*)sim->addr_to_mem(paddr);

riscv/processor.cc

@@ -199,9 +199,13 @@ void processor_t::set_privilege(reg_t prv)
 
 void processor_t::enter_debug_mode(uint8_t cause)
 {
+  fprintf(stderr, "enter_debug_mode(%d)\n", cause);
   state.dcsr.cause = cause;
+  state.dcsr.prv = state.prv;
+  state.prv = PRV_M;
   state.dpc = state.pc;
-  state.pc = DEBUG_ROM_ENTRY;
+  state.pc = DEBUG_ROM_START;
+  debug = true; // TODO
 }
 
 void processor_t::take_trap(trap_t& t, reg_t epc)
@@ -365,6 +369,7 @@ void processor_t::set_csr(int which, reg_t val)
     case CSR_MCAUSE: state.mcause = val; break;
     case CSR_MBADADDR: state.mbadaddr = val; break;
     case DCSR_ADDRESS:
+      // TODO: Use get_field style
       state.dcsr.prv = (val & DCSR_PRV_MASK) >> DCSR_PRV_OFFSET;
       state.dcsr.step = (val & DCSR_STEP_MASK) >> DCSR_STEP_OFFSET;
       // TODO: ndreset and fullreset

riscv/sim.cc

@@ -41,10 +41,17 @@ sim_t::sim_t(const char* isa, size_t nprocs, size_t mem_mb, bool halted,
     fprintf(stderr, "warning: only got %lu bytes of target mem (wanted %lu)\n",
             (unsigned long)memsz, (unsigned long)memsz0);
 
+  /* Copy Debug ROM into the end of the allocated block, because we surely
+   * didn't succeed in allocation 0xfffffffff800 bytes. */
+  /* TODO: Once everything uses the new memory map, just put this at the
+   * address that it actually belongs at. */
+  memcpy(mem + memsz - debug_rom_raw_len, debug_rom_raw, debug_rom_raw_len);
+
   debug_mmu = new mmu_t(this, NULL);
 
   for (size_t i = 0; i < procs.size(); i++) {
     procs[i] = new processor_t(isa, this, i);
+    if (halted)
       procs[i]->enter_debug_mode(DCSR_CAUSE_HALT);
   }