linux-kernel - Re: [RFC -tip 2/4] x86: uaccess: introduce __{get|put}

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Message-ID: <49507B88.1030407@ct.jp.nec.com>
Date:	Mon, 22 Dec 2008 21:47:52 -0800
From:	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
To:	Brian Gerst <brgerst@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC -tip 2/4] x86: uaccess: introduce __{get|put}_user exception
 handling framework

Brian Gerst wrote:
> On Tue, Dec 23, 2008 at 12:22 AM, Hiroshi Shimamoto
> <h-shimamoto@...jp.nec.com> wrote:
>> From: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
>>
>> Impact: introduce new framework
>>
>> Introduce exception handling framework.
>> __{get|put}_user_ex_try() begins exception block and
>> __{get|put}_user_ex_catch() ends block and if an exception occurred in this
>> block using __{get|put}_user_ex, direct jump to __{get|put}_user_ex_catch()
>> and err is set to specified value.
> 
> You shouldn't do this.  According to the gcc manual[1], "Speaking of
> labels, jumps from one asm to another are not supported. The
> compiler's optimizers do not know about these jumps, and therefore
> they cannot take account of them when deciding how to optimize."
> 
> [1] http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Extended-Asm.html
> 
thanks so much for this information!
I didn't know about this and it's what I want to know, thinking
about this series.

Thanks,
Hiroshi
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