Add kernel command line option for spike

It is very inconvenient to always embed kernel flat image into
OpenSBI for booting Linux/Xvisor on Spike.

We add optional "--kernel" command line option for spike. Using
this new option, users can specify kernel flat image separately
and OpenSBI ELF separately.

Signed-off-by: Anup Patel <anup.patel@wdc.com>

spike_main/spike.cc

@@ -50,6 +50,7 @@ static void help(int exit_code = 1)
   fprintf(stderr, "  --rbb-port=<port>     Listen on <port> for remote bitbang connection\n");
   fprintf(stderr, "  --dump-dts            Print device tree string and exit\n");
   fprintf(stderr, "  --disable-dtb         Don't write the device tree blob into memory\n");
+  fprintf(stderr, "  --kernel=<path>       Load kernel flat image into memory\n");
   fprintf(stderr, "  --initrd=<path>       Load kernel initrd into memory\n");
   fprintf(stderr, "  --bootargs=<args>     Provide custom bootargs for kernel [default: console=hvc0 earlycon=sbi]\n");
   fprintf(stderr, "  --real-time-clint     Increment clint time at real-time rate\n");
@@ -190,6 +191,8 @@ int main(int argc, char** argv)
   bool dtb_enabled = true;
   bool real_time_clint = false;
   size_t nprocs = 1;
+  const char* kernel = NULL;
+  reg_t kernel_offset, kernel_size;
   size_t initrd_size;
   reg_t initrd_start = 0, initrd_end = 0;
   const char* bootargs = NULL;
@@ -302,6 +305,7 @@ int main(int argc, char** argv)
   parser.option(0, "dump-dts", 0, [&](const char *s){dump_dts = true;});
   parser.option(0, "disable-dtb", 0, [&](const char *s){dtb_enabled = false;});
   parser.option(0, "dtb", 1, [&](const char *s){dtb_file = s;});
+  parser.option(0, "kernel", 1, [&](const char* s){kernel = s;});
   parser.option(0, "initrd", 1, [&](const char* s){initrd = s;});
   parser.option(0, "bootargs", 1, [&](const char* s){bootargs = s;});
   parser.option(0, "real-time-clint", 0, [&](const char *s){real_time_clint = true;});
@@ -341,6 +345,20 @@ int main(int argc, char** argv)
   if (!*argv1)
     help();
 
+  if (kernel && check_file_exists(kernel)) {
+    kernel_size = get_file_size(kernel);
+    if (isa[2] == '6' && isa[3] == '4')
+      kernel_offset = 0x200000;
+    else
+      kernel_offset = 0x400000;
+    for (auto& m : mems) {
+      if (kernel_size && (kernel_offset + kernel_size) < m.second->size()) {
+         read_file_bytes(kernel, 0, m.second->contents() + kernel_offset, kernel_size);
+         break;
+      }
+    }
+  }
+
   if (initrd && check_file_exists(initrd)) {
     initrd_size = get_file_size(initrd);
     for (auto& m : mems) {