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SubjectRe: [RFC] x86: bitops asm constraint fixes
On 03/13/2008 05:08 AM, Jan Beulich wrote:
> This (simplified) piece of code didn't behave as expected due to
> incorrect constraints in some of the bitops functions, when
> X86_FEATURE_xxx is referring to other than the first long:
>
> int test(struct cpuinfo_x86 *c) {
> if (cpu_has(c, X86_FEATURE_xxx))
> clear_cpu_cap(c, X86_FEATURE_xxx);
> return cpu_has(c, X86_FEATURE_xxx);
> }
>

This is a long-standing bug and your patch appears to fix it.

> --- linux-2.6.25-rc5/include/asm-x86/bitops.h 2008-03-10 13:24:33.000000000 +0100
> +++ 2.6.25-rc5-x86-clear-bit/include/asm-x86/bitops.h 2008-03-13 08:45:40.000000000 +0100
> @@ -24,9 +24,12 @@
> /* Technically wrong, but this avoids compilation errors on some gcc
> versions. */
> #define ADDR "=m" (*(volatile long *) addr)
> +#define BIT_ADDR "=m" (((volatile int *) addr)[nr >> 5])
> #else
> #define ADDR "+m" (*(volatile long *) addr)
> +#define BIT_ADDR "+m" (((volatile int *) addr)[nr >> 5])
> #endif
> +#define BASE_ADDR "m" (*(volatile int *) addr)

Can't you just do everything with unsigned longs, like this?

In include/asm-x86/types.h:

ifdef CONFIG_X86_32
# define BITS_PER_LONG 32
+# define BITMAP_ORDER 5
#else
# define BITS_PER_LONG 64
+# define BITMAP_ORDER 6
#endif

Then:

> #define ADDR "=m" (*(volatile long *) addr)
> +#define BIT_ADDR "=m" (((volatile long *) addr)[nr >> BITMAP_ORDER])
> #else
> #define ADDR "+m" (*(volatile long *) addr)
> +#define BIT_ADDR "+m" (((volatile long *) addr)[nr >> BITMAP_ORDER])
> #endif

No need for BASE_ADDR that way (or ADDR could be renamed to that.)


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