Checkpoint port of xv6 to x86-64. Passed usertests on 2 processors a few times.

The x86-64 doesn't just add two levels to page tables to support 64 bit
addresses, but is a different processor. For example, calling conventions,
system calls, and segmentation are different from 32-bit x86. Segmentation is
basically gone, but gs/fs in combination with MSRs can be used to hold a
per-core pointer. In general, x86-64 is more straightforward than 32-bit
x86. The port uses code from sv6 and the xv6 "rsc-amd64" branch.

A summary of the changes is as follows:

- Booting: switch to grub instead of xv6's bootloader (pass -kernel to qemu),
because xv6's boot loader doesn't understand 64bit ELF files.  And, we don't
care anymore about booting.

- Makefile: use -m64 instead of -m32 flag for gcc, delete boot loader, xv6.img,
bochs, and memfs. For now dont' use -O2, since usertests with -O2 is bigger than
MAXFILE!

- Update gdb.tmpl to be for i386 or x86-64

- Console/printf: use stdarg.h and treat 64-bit addresses different from ints
  (32-bit)

- Update elfhdr to be 64 bit

- entry.S/entryother.S: add code to switch to 64-bit mode: build a simple page
table in 32-bit mode before switching to 64-bit mode, share code for entering
boot processor and APs, and tweak boot gdt.  The boot gdt is the gdt that the
kernel proper also uses. (In 64-bit mode, the gdt/segmentation and task state
mostly disappear.)

- exec.c: fix passing argv (64-bit now instead of 32-bit).

- initcode.c: use syscall instead of int.

- kernel.ld: load kernel very high, in top terabyte.  64 bits is a lot of
address space!

- proc.c: initial return is through new syscall path instead of trapret.

- proc.h: update struct cpu to have some scratch space since syscall saves less
state than int, update struct context to reflect x86-64 calling conventions.

- swtch: simplify for x86-64 calling conventions.

- syscall: add fetcharg to handle x86-64 calling convetions (6 arguments are
passed through registers), and fetchaddr to read a 64-bit value from user space.

- sysfile: update to handle pointers from user space (e.g., sys_exec), which are
64 bits.

- trap.c: no special trap vector for sys calls, because x86-64 has a different
plan for system calls.

- trapasm: one plan for syscalls and one plan for traps (interrupt and
exceptions). On x86-64, the kernel is responsible for switching user/kernel
stacks. To do, xv6 keeps some scratch space in the cpu structure, and uses MSR
GS_KERN_BASE to point to the core's cpu structure (using swapgs).

- types.h: add uint64, and change pde_t to uint64

- usertests: exit() when fork fails, which helped in tracking down one of the
bugs in the switch from 32-bit to 64-bit

- vectors: update to make them 64 bits

- vm.c: use bootgdt in kernel too, program MSRs for syscalls and core-local
state (for swapgs), walk 4 levels in walkpgdir, add DEVSPACETOP, use task
segment to set kernel stack for interrupts (but simpler than in 32-bit mode),
add an extra argument to freevm (size of user part of address space) to avoid
checking all entries till KERNBASE (there are MANY TB before the top 1TB).

- x86: update trapframe to have 64-bit entries, which is what the processor
pushes on syscalls and traps.  simplify lgdt and lidt, using struct desctr,
which needs the gcc directives packed and aligned.

TODO:
- use int32 instead of int?
- simplify curproc(). xv6 has per-cpu state again, but this time it must have it.
- avoid repetition in walkpgdir
- fix validateint() in usertests.c
- fix bugs (e.g., observed one a case of entering kernel with invalid gs or proc

exec.c

@@ -4,6 +4,8 @@
 #include "mmu.h"
 #include "proc.h"
 #include "defs.h"
+#include "traps.h"
+#include "msr.h"
 #include "x86.h"
 #include "elf.h"
 
@@ -12,18 +14,18 @@ exec(char *path, char **argv)
 {
   char *s, *last;
   int i, off;
-  uint argc, sz, sp, ustack[3+MAXARG+1];
+  uint64 argc, sz, sp, ustack[3+MAXARG+1];
   struct elfhdr elf;
   struct inode *ip;
   struct proghdr ph;
   pde_t *pgdir, *oldpgdir;
   struct proc *curproc = myproc();
-
+  uint64 oldsz = curproc->sz;
+  
   begin_op();
 
   if((ip = namei(path)) == 0){
     end_op();
-    cprintf("exec: fail\n");
     return -1;
   }
   ilock(ip);
@@ -72,7 +74,7 @@ exec(char *path, char **argv)
   for(argc = 0; argv[argc]; argc++) {
     if(argc >= MAXARG)
       goto bad;
-    sp = (sp - (strlen(argv[argc]) + 1)) & ~3;
+    sp = (sp - (strlen(argv[argc]) + 1)) & ~(sizeof(uint64)-1);
     if(copyout(pgdir, sp, argv[argc], strlen(argv[argc]) + 1) < 0)
       goto bad;
     ustack[3+argc] = sp;
@@ -81,10 +83,13 @@ exec(char *path, char **argv)
 
   ustack[0] = 0xffffffff;  // fake return PC
   ustack[1] = argc;
-  ustack[2] = sp - (argc+1)*4;  // argv pointer
+  ustack[2] = sp - (argc+1)*sizeof(uint64);  // argv pointer
 
-  sp -= (3+argc+1) * 4;
-  if(copyout(pgdir, sp, ustack, (3+argc+1)*4) < 0)
+  curproc->tf->rdi = argc;
+  curproc->tf->rsi = sp - (argc+1)*sizeof(uint64);
+
+  sp -= (3+argc+1) * sizeof(uint64);
+  if(copyout(pgdir, sp, ustack, (3+argc+1)*sizeof(uint64)) < 0)
     goto bad;
 
   // Save program name for debugging.
@@ -92,20 +97,21 @@ exec(char *path, char **argv)
     if(*s == '/')
       last = s+1;
   safestrcpy(curproc->name, last, sizeof(curproc->name));
-
+    
   // Commit to the user image.
   oldpgdir = curproc->pgdir;
   curproc->pgdir = pgdir;
   curproc->sz = sz;
-  curproc->tf->eip = elf.entry;  // main
-  curproc->tf->esp = sp;
+  curproc->tf->rip = elf.entry;  // main
+  curproc->tf->rcx = elf.entry;
+  curproc->tf->rsp = sp;
   switchuvm(curproc);
-  freevm(oldpgdir);
+  freevm(oldpgdir, oldsz);
   return 0;
 
  bad:
   if(pgdir)
-    freevm(pgdir);
+    freevm(pgdir, sz);
   if(ip){
     iunlockput(ip);
     end_op();