On Tue, Feb 12, 2013 at 09:32:54AM -0800, Linus Torvalds wrote:
> On Tue, Feb 12, 2013 at 9:14 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> >
> > No, I think what he is talking about it this bit:
>
> Ok, I agree that the bitfield code actually looks cleaner.
>
> That said, maybe gcc has an easier time using a few odd builtins and
> magic typeof's. But at least the bitfield trick looks half-way
> portable..
I've just been trying hpa's solution on ARM, and I can't get it to work,
because the compiler refuses to put the struct { unsigned long long ... }
into the register(s) we need for the out of line assembly:
#define get_user(x,p) \
({ \
register const typeof(*(p)) __user *__p asm("r0") = (p);\
register int __e asm("r0"); \
register struct { \
unsigned long long __r2:8 * sizeof(*(__p)); \
} __v asm("r2"); \
switch (sizeof(*(__p))) { \
case 1: \
__get_user_x(__v.__r2, __p, __e, 1, "lr"); \
break; \
case 2: \
__get_user_x(__v.__r2, __p, __e, 2, "r3", "lr");\
break; \
case 4: \
__get_user_x(__v.__r2, __p, __e, 4, "lr"); \
break; \
case 8: \
__get_user_x(__v.__r2, __p, __e, 8, "lr"); \
break; \
default: __e = __get_user_bad(); break; \
} \
x = (typeof(*(__p))) __v.__r2; \
__e; \
})
This ends up with __v.__r2 ending up in r1/(r2) not r2/(r3).
However, I do have a working solution for 32-bit ARM which seems to work
fine with my test cases here, though as I mentioned to hpa, it may not
be portable to other 32-bit architectures:
#ifdef BIG_ENDIAN
#define __get_user_xb(__r2,__p,__e,__s,__i...) \
__get_user_x(__r2,(unsigned)__p+4,__e,__s,__i)
#else
#define __get_user_xb __get_user_x
#endif
#define get_user(x,p) \
({ \
register const typeof(*(p)) __user *__p asm("r0") = (p);\
register int __e asm("r0"); \
register typeof(x) __r2 asm("r2"); \
switch (sizeof(*(__p))) { \
case 1: \
__get_user_x(__r2, __p, __e, 1, "lr"); \
break; \
case 2: \
__get_user_x(__r2, __p, __e, 2, "r3", "lr"); \
break; \
case 4: \
__get_user_x(__r2, __p, __e, 4, "lr"); \
break; \
case 8: \
{ \
if (sizeof((x)) < 8) \
__get_user_xb(__r2, __p, __e, 4, "lr"); \
else \
__get_user_x(__r2, __p, __e, 8, "lr"); \
} \
break; \
default: __e = __get_user_bad(); break; \
} \
x = (typeof(*(__p))) __r2; \
__e; \
})
It's risky because it relies upon a "register" being allocated as 32-bits
even if typeof(x) is 8-bit or 16-bit.
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