linux-kernel - Re: [patch] i386: remove IOPL check on task switch

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Date:	Fri, 03 Nov 2006 10:06:04 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Chuck Ebbert <76306.1226@...puserve.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@...e.de>
Subject: Re: [patch] i386: remove IOPL check on task switch

Chuck Ebbert wrote:
> IOPL is implicitly saved and restored on task switch,
> so explicit check is no longer needed.
>
> Signed-off-by: Chuck Ebbert <76306.1226@...puserve.com>
>
> --- 2.6.19-rc4-32smp.orig/arch/i386/kernel/process.c
> +++ 2.6.19-rc4-32smp/arch/i386/kernel/process.c
> @@ -681,12 +681,6 @@ struct task_struct fastcall * __switch_t
>  		loadsegment(gs, next->gs);
>  
>  	/*
> -	 * Restore IOPL if needed.
> -	 */
> -	if (unlikely(prev->iopl != next->iopl))
> -		set_iopl_mask(next->iopl);
> -
> -	/*
>  	 * Now maybe handle debug registers and/or IO bitmaps
>  	 */
>  	if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW)
>   

Nack.  This is used for paravirt-ops kernels that use IOPL'd userspace.  
Fixing it would require a fairly heavy penalty on the iret path, since 
every single instruction there contributes to a critical region which 
must have custom fixup code, or some other technique to provide 
protection against interrupt re-entrancy.

At least, let's discuss other potential solutions first - for now it is 
harmless.

Zach
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