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


From: Fabrice Bellard
Subject: [Qemu-devel] Re: qemu 0.5.1 emulation bug?
Date: Sat, 24 Jan 2004 17:30:47 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030624

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.








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