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

[Herbert Poetzl]

On Sun, Jan 25, 2004 at 01:09:52AM +1100, Rusty Russell wrote:
> Just spent two hours chasing down why recent 2.6.2-rc1 kernels don't
> boot.

hum, interesting, 2.6.2-rc1 is booting here, with 
the 'to be tested' cvs version ... (5.2 rc)

best,
Herbert

> 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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