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

[Jeremy Fitzhardinge <jeremy@goop.org>]

Andi Kleen wrote:
>>  
>> -	/* 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.
>   

Yep.  And the "m" args in the pda asm isn't quite right for rmw PDA ops 
(not that there are any at the moment).

>> +1:	movw GS(%esp), %gs
>>     
>
> movl is recommended in 32bit mode
>   

OK.  I thought the assembler objected to me about it.

>> --- 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.
>   

The sigcontext contains the userspace register state at the time of the 
signal.  Since userspace %gs is stored in the on-stack pt_regs, that 
should be where it fetches it from to fill out the sigcontext, rather 
than the kernel's internal value of %gs - in other words, it should be 
the same as ds and es.  Or am I missing 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.
>   

Yep.  But there seems to be some other very dubious code in there as 
well (the asm("mov %%eax,%0" : "=r" (eax)) sequence).  I was wondering 
about what it all does...

    J