Re: [PATCH 20/30] microblaze_mmu_v1: uaccess MMU update

Arnd Bergmann
2009-04-29T15:37:27Z

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.