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

1 files changed, 49 insertions, 9 deletions

diff --git a/syscall.c b/syscall.c
index ee85261..3ffe3d8 100644
--- a/syscall.c
+++ b/syscall.c
@@ -15,13 +15,13 @@
 
 // Fetch the int at addr from the current process.
 int
-fetchint(uint addr, int *ip)
+fetchint(uint64 addr, int *ip)
 {
   struct proc *curproc = myproc();
 
   if(addr >= curproc->sz || addr+4 > curproc->sz)
     return -1;
-  *ip = *(int*)(addr);
+  *ip = *(uint64*)(addr);
   return 0;
 }
 
@@ -29,7 +29,7 @@ fetchint(uint addr, int *ip)
 // Doesn't actually copy the string - just sets *pp to point at it.
 // Returns length of string, not including nul.
 int
-fetchstr(uint addr, char **pp)
+fetchstr(uint64 addr, char **pp)
 {
   char *s, *ep;
   struct proc *curproc = myproc();
@@ -45,11 +45,51 @@ fetchstr(uint addr, char **pp)
   return -1;
 }
 
+static uint64
+fetcharg(int n)
+{
+  struct proc *curproc = myproc();
+  switch (n) {
+  case 0:
+    return curproc->tf->rdi;
+  case 1:
+    return curproc->tf->rsi;
+  case 2:
+    return curproc->tf->rdx;
+  case 3:
+    return curproc->tf->r10;
+  case 4:
+    return curproc->tf->r8;
+  case 5:
+    return curproc->tf->r9;
+  }
+  panic("fetcharg");
+  return -1;
+}
+
+int
+fetchaddr(uint64 addr, uint64 *ip)
+{
+  struct proc *curproc = myproc();
+  if(addr >= curproc->sz || addr+sizeof(uint64) > curproc->sz)
+    return -1;
+  *ip = *(uint64*)(addr);
+  return 0;
+}
+
 // Fetch the nth 32-bit system call argument.
 int
 argint(int n, int *ip)
 {
-  return fetchint((myproc()->tf->esp) + 4 + 4*n, ip);
+  *ip = fetcharg(n);
+  return 0;
+}
+
+int
+argaddr(int n, uint64 *ip)
+{
+  *ip = fetcharg(n);
+  return 0;
 }
 
 // Fetch the nth word-sized system call argument as a pointer
@@ -58,10 +98,10 @@ argint(int n, int *ip)
 int
 argptr(int n, char **pp, int size)
 {
-  int i;
+  uint64 i;
   struct proc *curproc = myproc();
  
-  if(argint(n, &i) < 0)
+  if(argaddr(n, &i) < 0)
     return -1;
   if(size < 0 || (uint)i >= curproc->sz || (uint)i+size > curproc->sz)
     return -1;
@@ -134,12 +174,12 @@ syscall(void)
   int num;
   struct proc *curproc = myproc();
 
-  num = curproc->tf->eax;
+  num = curproc->tf->rax;
   if(num > 0 && num < NELEM(syscalls) && syscalls[num]) {
-    curproc->tf->eax = syscalls[num]();
+    curproc->tf->rax = syscalls[num]();
   } else {
     cprintf("%d %s: unknown sys call %d\n",
             curproc->pid, curproc->name, num);
-    curproc->tf->eax = -1;
+    curproc->tf->rax = -1;
   }
 }