From: "Guan Xuetao" <gxt@mprc.pku.edu.cn>
To: "'Arnd Bergmann'" <arnd@arndb.de>
Cc: <linux-kernel@vger.kernel.org>, <linux-arch@vger.kernel.org>,
        "'Greg KH'" <greg@kroah.com>
References: <014b01cbcdad$f110afb0$d3320f10$@mprc.pku.edu.cn> <201102171845.01177.arnd@arndb.de>
In-Reply-To: <201102171845.01177.arnd@arndb.de>
Subject: RE: [PATCH 05/12] unicore32 additional architecture files: low-level lib: uaccess
Date: Sun, 27 Feb 2011 11:21:12 +0800
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> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@arndb.de]
> Sent: Friday, February 18, 2011 1:45 AM
> To: Guan Xuetao
> Cc: linux-kernel@vger.kernel.org; linux-arch@vger.kernel.org; 'Greg KH'
> Subject: Re: [PATCH 05/12] unicore32 additional architecture files: low-level lib: uaccess
> 
> On Wednesday 16 February 2011, Guan Xuetao wrote:
> > +
> > +#define __kernel_ok		(segment_eq(get_fs(), KERNEL_DS))
> > +#define __user_ok(addr, size)	(((size) <= TASK_SIZE)			\
> > +				&& ((addr) <= TASK_SIZE - (size)))
> > +#define __access_ok(addr, size)	(__kernel_ok || __user_ok((addr), (size)))
> 
> A nice trick to simplify this is to introduce a per-process variable for comparing
> the pointer, set_fs() then sets this variable to either TASK_SIZE or
> UINT_MAX.

Perhaps like this:
1. add a member named kaddr in thread_info
2. add following assignment codes in set_fs:
        current_thread_info()->kaddr =  segment_eq(fs, KERNEL_DS) ? UINT_MAX : TASK_SIZE;
3. then __access_ok will be:
        #define addr 
        #define __access_ok(addr, size)   (((size) <= TASK_SIZE)			\
				&& ((addr) <= current_thread_info()->kaddr)  - (size)))

> 
> This saves one conditional branch for each call to __access_ok.
> 
> 	Arnd

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