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

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Date:	Sun, 27 Aug 2006 17:57:18 +0200
From:	Andi Kleen <ak@...e.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	linux-kernel@...r.kernel.org,
	Chuck Ebbert <76306.1226@...puserve.com>,
	Zachary Amsden <zach@...are.com>,
	Jan Beulich <jbeulich@...ell.com>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH RFC 3/6] Use %gs as the PDA base-segment in the kernel.


>  
> -	/* 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
-
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