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, 12 insertions, 45 deletions

diff --git a/entryother.S b/entryother.S
index a3b6dc2..3e502f3 100644
--- a/entryother.S
+++ b/entryother.S
@@ -13,11 +13,9 @@
 #
 # Startothers (in main.c) sends the STARTUPs one at a time.
 # It copies this code (start) at 0x7000.  It puts the address of
-# a newly allocated per-core stack in start-4,the address of the
-# place to jump to (mpenter) in start-8, and the physical address
+# a newly allocated per-core stack in start-12,the address of the
+# place to jump to (apstart32) in start-4, and the physical address
 # of entrypgdir in start-12.
-#
-# This code combines elements of bootasm.S and entry.S.
 
 .code16           
 .globl start
@@ -41,53 +39,22 @@ start:
   # Complete the transition to 32-bit protected mode by using a long jmp
   # to reload %cs and %eip.  The segment descriptors are set up with no
   # translation, so that the mapping is still the identity mapping.
-  ljmpl    $(SEG_KCODE<<3), $(start32)
+  ljmpl    $(KCSEG32), $start32
 
-//PAGEBREAK!
-.code32  # Tell assembler to generate 32-bit code now.
+.code32
 start32:
-  # Set up the protected-mode data segment registers
-  movw    $(SEG_KDATA<<3), %ax    # Our data segment selector
-  movw    %ax, %ds                # -> DS: Data Segment
-  movw    %ax, %es                # -> ES: Extra Segment
-  movw    %ax, %ss                # -> SS: Stack Segment
-  movw    $0, %ax                 # Zero segments not ready for use
-  movw    %ax, %fs                # -> FS
-  movw    %ax, %gs                # -> GS
-
-  # Turn on page size extension for 4Mbyte pages
-  movl    %cr4, %eax
-  orl     $(CR4_PSE), %eax
-  movl    %eax, %cr4
-  # Use entrypgdir as our initial page table
-  movl    (start-12), %eax
-  movl    %eax, %cr3
-  # Turn on paging.
-  movl    %cr0, %eax
-  orl     $(CR0_PE|CR0_PG|CR0_WP), %eax
-  movl    %eax, %cr0
+ movl $start-12, %esp
+ movl start-4, %ecx
+ jmp *%ecx
 
-  # Switch to the stack allocated by startothers()
-  movl    (start-4), %esp
-  # Call mpenter()
-  call	 *(start-8)
-
-  movw    $0x8a00, %ax
-  movw    %ax, %dx
-  outw    %ax, %dx
-  movw    $0x8ae0, %ax
-  outw    %ax, %dx
-spin:
-  jmp     spin
-
-.p2align 2
+.align 4
 gdt:
   SEG_NULLASM
-  SEG_ASM(STA_X|STA_R, 0, 0xffffffff)
-  SEG_ASM(STA_W, 0, 0xffffffff)
-
+  SEG_ASM(0xa, 0, 0xffffffff)
+  SEG_ASM(0x2, 0, 0xffffffff)
 
+.align  16
 gdtdesc:
-  .word   (gdtdesc - gdt - 1)
+  .word   0x17 # sizeof(gdt)-1
   .long   gdt