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, 28 insertions, 41 deletions

diff --git a/main.c b/main.c
index 9924e64..449396a 100644
--- a/main.c
+++ b/main.c
@@ -6,17 +6,22 @@
 #include "proc.h"
 #include "x86.h"
 
-static void startothers(void);
-static void mpmain(void)  __attribute__((noreturn));
 extern pde_t *kpgdir;
 extern char end[]; // first address after kernel loaded from ELF file
 
+static void main(void)  __attribute__((noreturn));
+static void startothers(void);
+
+
 // Bootstrap processor starts running C code here.
 // Allocate a real stack and switch to it, first
 // doing some setup required for memory allocator to work.
 int
-main(void)
+bpmain(uint64 mbmagic, uint64 mbaddr)
 {
+  if(mbmagic != 0x2badb002)
+       panic("multiboot header not found");
+
   kinit1(end, P2V(4*1024*1024)); // phys page allocator
   kvmalloc();      // kernel page table
   mpinit();        // detect other processors
@@ -30,26 +35,19 @@ main(void)
   tvinit();        // trap vectors
   binit();         // buffer cache
   fileinit();      // file table
-  ideinit();       // disk 
+  ideinit();       // disk
+  
   startothers();   // start other processors
+ 
   kinit2(P2V(4*1024*1024), P2V(PHYSTOP)); // must come after startothers()
   userinit();      // first user process
-  mpmain();        // finish this processor's setup
-}
-
-// Other CPUs jump here from entryother.S.
-static void
-mpenter(void)
-{
-  switchkvm();
-  seginit();
-  lapicinit();
-  mpmain();
+  main();        
+  return 0;
 }
 
 // Common CPU setup code.
 static void
-mpmain(void)
+main(void)
 {
   cprintf("cpu%d: starting %d\n", cpuid(), cpuid());
   idtinit();       // load idt register
@@ -57,7 +55,17 @@ mpmain(void)
   scheduler();     // start running processes
 }
 
-pde_t entrypgdir[];  // For entry.S
+// Other CPUs jump here from entryother.S.
+void
+apmain(void)
+{
+  switchkvm();
+  seginit();
+  lapicinit();
+  main();
+}
+
+void apstart(void);
 
 // Start the non-boot (AP) processors.
 static void
@@ -72,7 +80,7 @@ startothers(void)
   // The linker has placed the image of entryother.S in
   // _binary_entryother_start.
   code = P2V(0x7000);
-  memmove(code, _binary_entryother_start, (uint)_binary_entryother_size);
+  memmove(code, _binary_entryother_start, (uint64)_binary_entryother_size);
 
   for(c = cpus; c < cpus+ncpu; c++){
     if(c == mycpu())  // We've started already.
@@ -82,9 +90,8 @@ startothers(void)
     // pgdir to use. We cannot use kpgdir yet, because the AP processor
     // is running in low  memory, so we use entrypgdir for the APs too.
     stack = kalloc();
-    *(void**)(code-4) = stack + KSTACKSIZE;
-    *(void(**)(void))(code-8) = mpenter;
-    *(int**)(code-12) = (void *) V2P(entrypgdir);
+    *(uint32*)(code-4) = V2P(apstart);
+    *(uint64*)(code-12) = (uint64) (stack+KSTACKSIZE);
 
     lapicstartap(c->apicid, V2P(code));
 
@@ -94,23 +101,3 @@ startothers(void)
   }
 }
 
-// The boot page table used in entry.S and entryother.S.
-// Page directories (and page tables) must start on page boundaries,
-// hence the __aligned__ attribute.
-// PTE_PS in a page directory entry enables 4Mbyte pages.
-
-__attribute__((__aligned__(PGSIZE)))
-pde_t entrypgdir[NPDENTRIES] = {
-  // Map VA's [0, 4MB) to PA's [0, 4MB)
-  [0] = (0) | PTE_P | PTE_W | PTE_PS,
-  // Map VA's [KERNBASE, KERNBASE+4MB) to PA's [0, 4MB)
-  [KERNBASE>>PDXSHIFT] = (0) | PTE_P | PTE_W | PTE_PS,
-};
-
-//PAGEBREAK!
-// Blank page.
-//PAGEBREAK!
-// Blank page.
-//PAGEBREAK!
-// Blank page.
-