Re: [Qemu-devel] [PATCH 1/6] bochs: use pread

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/04/2010 05:44 AM, Christoph Hellwig wrote:
> Use pread instead of lseek + read in preparation of using the qemu
> block API.
>
> Signed-off-by: Christoph Hellwig<hch@lst.de>
>    

Looks good to me.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>

Regards,

Anthony Liguori

> Index: qemu-kevin/block/bochs.c
> ===================================================================
> --- qemu-kevin.orig/block/bochs.c	2010-05-03 12:58:53.419012621 +0200
> +++ qemu-kevin/block/bochs.c	2010-05-03 12:59:13.873005360 +0200
> @@ -125,7 +125,7 @@ static int bochs_open(BlockDriverState *
>
>       s->fd = fd;
>
> -    if (read(fd,&bochs, sizeof(bochs)) != sizeof(bochs)) {
> +    if (pread(fd,&bochs, sizeof(bochs), 0) != sizeof(bochs)) {
>           goto fail;
>       }
>
> @@ -144,14 +144,10 @@ static int bochs_open(BlockDriverState *
>         bs->total_sectors = le64_to_cpu(bochs.extra.redolog.disk) / 512;
>       }
>
> -    if (lseek(s->fd, le32_to_cpu(bochs.header), SEEK_SET) == (off_t)-1) {
> -        goto fail;
> -    }
> -
>       s->catalog_size = le32_to_cpu(bochs.extra.redolog.catalog);
>       s->catalog_bitmap = qemu_malloc(s->catalog_size * 4);
> -    if (read(s->fd, s->catalog_bitmap, s->catalog_size * 4) !=
> -	s->catalog_size * 4)
> +    if (pread(s->fd, s->catalog_bitmap, s->catalog_size * 4,
> +              le32_to_cpu(bochs.header)) != s->catalog_size * 4)
>   	goto fail;
>       for (i = 0; i<  s->catalog_size; i++)
>   	le32_to_cpus(&s->catalog_bitmap[i]);
> @@ -169,54 +165,35 @@ static int bochs_open(BlockDriverState *
>       return -1;
>   }
>
> -static inline int seek_to_sector(BlockDriverState *bs, int64_t sector_num)
> +static int64_t seek_to_sector(BlockDriverState *bs, int64_t sector_num)
>   {
>       BDRVBochsState *s = bs->opaque;
>       int64_t offset = sector_num * 512;
> -    int64_t extent_index, extent_offset, bitmap_offset, block_offset;
> +    int64_t extent_index, extent_offset, bitmap_offset;
>       char bitmap_entry;
>
>       // seek to sector
>       extent_index = offset / s->extent_size;
>       extent_offset = (offset % s->extent_size) / 512;
>
> -    if (s->catalog_bitmap[extent_index] == 0xffffffff)
> -    {
> -//	fprintf(stderr, "page not allocated [%x - %x:%x]\n",
> -//	    sector_num, extent_index, extent_offset);
> -	return -1; // not allocated
> +    if (s->catalog_bitmap[extent_index] == 0xffffffff) {
> +	return -1; /* not allocated */
>       }
>
>       bitmap_offset = s->data_offset + (512 * s->catalog_bitmap[extent_index] *
>   	(s->extent_blocks + s->bitmap_blocks));
> -    block_offset = bitmap_offset + (512 * (s->bitmap_blocks + extent_offset));
>
> -//    fprintf(stderr, "sect: %x [ext i: %x o: %x] ->  %x bitmap: %x block: %x\n",
> -//	sector_num, extent_index, extent_offset,
> -//	le32_to_cpu(s->catalog_bitmap[extent_index]),
> -//	bitmap_offset, block_offset);
> -
> -    // read in bitmap for current extent
> -    if (lseek(s->fd, bitmap_offset + (extent_offset / 8), SEEK_SET) ==
> -        (off_t)-1) {
> +    /* read in bitmap for current extent */
> +    if (pread(s->fd,&bitmap_entry, 1, bitmap_offset + (extent_offset / 8))
> +            != 1) {
>           return -1;
>       }
>
> -    if (read(s->fd,&bitmap_entry, 1) != 1)
> -        return -1;
> -
> -    if (!((bitmap_entry>>  (extent_offset % 8))&  1))
> -    {
> -//	fprintf(stderr, "sector (%x) in bitmap not allocated\n",
> -//	    sector_num);
> -	return -1; // not allocated
> +    if (!((bitmap_entry>>  (extent_offset % 8))&  1)) {
> +	return -1; /* not allocated */
>       }
>
> -    if (lseek(s->fd, block_offset, SEEK_SET) == (off_t)-1) {
> -        return -1;
> -    }
> -
> -    return 0;
> +    return bitmap_offset + (512 * (s->bitmap_blocks + extent_offset));
>   }
>
>   static int bochs_read(BlockDriverState *bs, int64_t sector_num,
> @@ -226,13 +203,13 @@ static int bochs_read(BlockDriverState *
>       int ret;
>
>       while (nb_sectors>  0) {
> -	if (!seek_to_sector(bs, sector_num))
> -	{
> -	    ret = read(s->fd, buf, 512);
> -	    if (ret != 512)
> -		return -1;
> -	}
> -	else
> +        int64_t block_offset = seek_to_sector(bs, sector_num);
> +        if (block_offset>= 0) {
> +            ret = pread(s->fd, buf, 512, block_offset);
> +            if (ret != 512) {
> +                return -1;
> +            }
> +        } else
>               memset(buf, 0, 512);
>           nb_sectors--;
>           sector_num++;
>
>
>
>