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

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Date:	Wed, 29 Apr 2009 17:35:28 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	monstr@...str.eu
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

On Wednesday 29 April 2009, Michal Simek wrote:
> > 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. 

Well, you can probably do something like this:

#ifdef CONFIG_MMU
#define __FIXUP_SECTION		".section .fixup,\"ax\"\n"
#define __EX_TABLE_SECTION	".section __ex_table,\"a\"\n"
#else
#define __FIXUP_SECTION		".section .discard,\"ax\"\n"
#define __EX_TABLE_SECTION	".section .discard,\"a\"\n"
#endif

+#define __get_user_asm(insn, __gu_ptr, __gu_val, __gu_err)             \
+({                                                                     \
+       __asm__ __volatile__ (                                          \
+                       "1:"    insn    " %1, %2, r0; \n"               \
+                       "       addk    %0, r0, r0;   \n"               \
+                       "2:                           \n"               \
+                        __FIXUP_SECTION				\
+                       "3:      brid    2b	      \n"               \
+                       "        addik   %0, r0, %3   \n"               \
+                       ".previous		      \n"               \
+                        __EX_TABLE_SECTION	                        \
+                       ".word   1b,3b		      \n"               \
+                       ".previous		      \n"               \
+               : "=r"(__gu_err), "=r"(__gu_val)                        \
+               : "r"(__gu_ptr), "i"(-EFAULT)                           \
+       );                                                              \
+})

This should simply throw away all the fixups if you list the .discard
section in vmlinux.lds.S under '/DISCARD/:', so you have no
overhead at all.

	Arnd <><
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