Re: [PATCH RFC 3/6] Use %gs as the PDA base-segment in the kernel.

Andi Kleen
2006-08-27T16:02:09Z

>
> - /* Clear %fs and %gs. */
> - asm volatile ("movl %0, %%fs; movl %0, %%gs" : : "r" (0));
> + /* Clear %fs. */
> + asm volatile ("mov %0, %%fs" : : "r" (0));
> +
> + /* Set %gs for this CPU's PDA */
> + asm volatile ("mov %0, %%gs" : : "r" (__KERNEL_PDA));

I would add memory clobbers here to make sure the dependency on read/write pda
is right.

> +1: movw GS(%esp), %gs

movl is recommended in 32bit mode

> --- a/arch/i386/kernel/signal.c
> +++ b/arch/i386/kernel/signal.c
> @@ -128,7 +128,7 @@ restore_sigcontext(struct pt_regs *regs,
> X86_EFLAGS_TF | X86_EFLAGS_SF | X86_EFLAGS_ZF | \
> X86_EFLAGS_AF | X86_EFLAGS_PF | X86_EFLAGS_CF)
>
> - GET_SEG(gs);
> + COPY_SEG(gs);
> GET_SEG(fs);
> COPY_SEG(es);
> COPY_SEG(ds);
> @@ -244,9 +244,7 @@ setup_sigcontext(struct sigcontext __use
> {
> int tmp, err = 0;
>
> - tmp = 0;
> - savesegment(gs, tmp);
> - err |= __put_user(tmp, (unsigned int __user *)&sc->gs);
> + err |= __put_user(regs->xgs, (unsigned int __user *)&sc->gs);
> savesegment(fs, tmp);
> err |= __put_user(tmp, (unsigned int __user *)&sc->fs);

Hmm, changing it for the sc looks a bit bogus. If everything
is right nothing should change for user space, but this changes something.

> @@ -306,7 +306,7 @@ static void do_sys_vm86(struct kernel_vm
> tsk->thread.screen_bitmap = info->screen_bitmap;
> if (info->flags & VM86_SCREEN_BITMAP)
> mark_screen_rdonly(tsk->mm);
> - __asm__ __volatile__("xorl %eax,%eax; movl %eax,%fs; movl %eax,%gs\n\t");
> + __asm__ __volatile__("movl %0,%%fs\n\t" : : "r" (0));

This is actually a useful bug fix on its own.

-Andi