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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
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mycpu() in proc.c.
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to find a per-cpu id with which we locate a cpu's cpu struct.
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myproc() points to a different thread.
myproc();
sched();
myproc(); // this proc maybe different than the one before sched
Thus, in a function that operates on one thread better to retrieve the
current process once at the start of the function.
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fix bugs where caller incorrectly released lock on error return path.
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for f in *.{h,c}; do sed -i .sed 's/[[:blank:]]*$//' $f; done
(Thanks to Nicolás Wolovick)
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however, processes still modify their own p->* without locking
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- move log into metadata part of disk, so that marking
that the log's blocks are in use falls out for free
- superblock describes the whole disk (sizes and offets)
- sizes and offsets are computed in one place (mkfs) and
the rest of the code refers to the superblock for these values,
instead of recomputing them.
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for bug reported by [email protected] and [email protected]
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exec() with arguments that don't fit on a single page.
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Invoke initlog from forkret on first user process
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Allocate proper kernel page table immediately in main using boot allocator
Remove pginit
Simplify address space layout a tiny bit
More to come (e.g., superpages to simplify static table)
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Very important to give qemu memory through PHYSTOP :(
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Variable declarations at top of function,
separate from initialization.
Use == 0 instead of ! for checking pointers.
Consistent spacing around {, *, casts.
Declare 0-parameter functions as (void) not ().
Integer valued functions return -1 on failure, 0 on success.
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delete most comments from bootother.S (since copy of bootasm.S)
ksegment() -> seginit()
move more stuff from main() to mainc()
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makes their interface match up better with proc->sz and also simplifies the callers (it even gets the main body of exec on one page).
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They also make sense in this order, so it's not just for page layout.
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do not keep sorted contiguous free list
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put an invalid page below the stack
have fork() handle invalid pages
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fix corner cases with alignment when mapping kernel ELF file
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Includes code for TLB shootdown (which actually seems unnecessary for xv6)
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this time do it ourselves instead of piggybacking on TLS.
add -fno-pic to Makefile; pic code breaks our fake TLS.
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still not quite but a lot better.
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* rename c/cp to cpu/proc
* rename cpu.context to cpu.scheduler
* fix some comments
* formatting for printout
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pdf has very good page breaks now.
would be a good copy for fall 2009.
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