linux-kernel - Re: [PATCH 20/30] microblaze_mmu

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Date:	Wed, 29 Apr 2009 17:19:37 +0200
From:	Michal Simek <monstr@...str.eu>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, john.williams@...alogix.com
Subject: Re: [PATCH 20/30] microblaze_mmu_v1: uaccess MMU update

Arnd Bergmann wrote:
> On Wednesday 29 April 2009, Michal Simek wrote:
>> Here is that change which remove address space problem.
>>
>> #define __clear_user(addr, n)   (memset((__force void *)(addr), 0, (n)), 0)
>>
>> Am I right?
>>
>> The same mishmash is for memset_fromio/memset_toio and maybe some others which I want to fix too.
> 
> This will work, but a better fix would be to define an inline
> function that explicitly takes a __user pointer. This would give
> you warnings when code accidentally calls __clear_user on a
> kernel pointer (this also adds the might_sleep()):
> 
> static inline unsigned long __must_check 
> __clear_user(void __user *to, unsigned long n)
> {
> 	memset((__force void *)addr, 0, n);
> 	return 0;
> }
> 
> static inline unsigned long __must_check 
> clear_user(void __user *to, unsigned long n)
> {
> 	might_sleep();
> 	if (unlikely(!access_ok(VERIFY_WRITE, to, n)))
> 		return n;
> 
> 	return __clear_user(to, n);
> }
> 
> The above is just the nommu variant. For mmu, you need to
> have exception handling in __clear_user to take care of the
> case where the address is part of the user mapping (access_ok)
> but not currently mapped.

Is it possible to do it for noMMU kernel too? I mean current MMU implementation of
__clear_user in asm is faster than call memset for noMMU. I think I can use MMU implementation for
noMMU too. Add two words to __ex_table
just extend size of one section but not too much and won't be used for noMMU.

Michal


> 
> 	Arnd <><


-- 
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
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