Make the ELF entry point a physical address

This way, the bootloader doesn't have to translate the entry point. This also makes xv6 multiboot-compliant and follows the convention used by Linux.

Make the ELF entry point a physical address
2fc6c35b · Austin Clements · 68b58727 · 2fc6c35b · 2fc6c35b
--- a/bootmain.c
+++ b/bootmain.c
@@ -43,7 +43,7 @@ bootmain(void)
  // Call the entry point from the ELF header.
  // Does not return!
-  entry = (void(*)(void))(elf->entry - KERNBASE);
+  entry = (void(*)(void))(elf->entry);
  entry();
 }

--- a/entry.S
+++ b/entry.S
@@ -25,15 +25,16 @@
 .globl multiboot_header
 multiboot_header:
  #define magic 0x1badb002
-  #define flags (1<<16 | 1<<0)
+  #define flags 0
  .long magic
  .long flags
  .long (-magic-flags)
-  .long multiboot_header  # beginning of image
-  .long multiboot_header
+# By convention, the _start symbol specifies the ELF entry point.
-  .long edata
+# Since we haven't set up virtual memory yet, our entry point is
-  .long end
+# the physical address of 'entry'.
-  .long entry
+.globl _start
+_start = V2P_WO(entry)
 # Entering xv6 on boot processor.  Machine is mostly set up.
 .globl entry