[Qemu-devel] Re: qemu 0.5.1 emulation bug?

[Fabrice Bellard <fabrice.bellard@free.fr>]

Thanx for the report. It should work now (eflags optimisation bug with 
string operations).

Fabrice.

Rusty Russell wrote:
> Just spent two hours chasing down why recent 2.6.2-rc1 kernels don't
> boot.
> 
> It turns out that find_next_bit() returns 0 under "qemu-fast" and
> "qemu" where it returns 32 under "qemu-i386" and native x86.
> 
> Bedtime for me, but here's the offending code:
> 
> /**
>  * find_first_bit - find the first set bit in a memory region
>  * @addr: The address to start the search at
>  * @size: The maximum size to search
>  *
>  * Returns the bit-number of the first set bit, not the number of the byte
>  * containing a bit.
>  */
> static __inline__ int xfind_first_bit(const unsigned long *addr, unsigned size)
> {
> 	int d0, d1;
> 	int res;
> 
> 	/* This looks at memory. Mark it volatile to tell gcc not to move it around */
> 	__asm__ __volatile__(
> 		"xorl %%eax,%%eax\n\t"
> 		"repe; scasl\n\t"
> 		"jz 1f\n\t"
> 		"leal -4(%%edi),%%edi\n\t"
> 		"bsfl (%%edi),%%eax\n"
> 		"1:\tsubl %%ebx,%%edi\n\t"
> 		"shll $3,%%edi\n\t"
> 		"addl %%edi,%%eax"
> 		:"=a" (res), "=&c" (d0), "=&D" (d1)
> 		:"1" ((size + 31) >> 5), "2" (addr), "b" (addr));
> 	return res;
> }
> 
> /**
>  * find_next_bit - find the first set bit in a memory region
>  * @addr: The address to base the search on
>  * @offset: The bitnumber to start searching at
>  * @size: The maximum size to search
>  */
> static __inline__ int xfind_next_bit(const unsigned long *addr, int size, int offset)
> {
> 	const unsigned long *p = addr + (offset >> 5);
> 	int set = 0, bit = offset & 31, res;
> 
> 	if (bit) {
> 		/*
> 		 * Look for nonzero in the first 32 bits:
> 		 */
> 		__asm__("bsfl %1,%0\n\t"
> 			"jne 1f\n\t"
> 			"movl $32, %0\n"
> 			"1:"
> 			: "=r" (set)
> 			: "r" (*p >> bit));
> 		if (set < (32 - bit))
> 			return set + offset;
> 		set = 32 - bit;
> 		p++;
> 	}
> 	/*
> 	 * No set bit yet, search remaining full words for a bit
> 	 */
> 	res = xfind_first_bit (p, size - 32 * (p - addr));
> 	return (offset + set + res);
> }
> 
> int main()
> {
> 	unsigned long map = 1;
> 
> 	printf("find_next_bit of %lu = %i\n",
> 	       map, xfind_next_bit(&map, 32, 1));
> 	return 0;
> }
> 
> 
> --
>   Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
> 
>