xio_start_offset
xbd_io->xio_start_offset = xbd_io->xio_seg[0].first_sect * VBD_BSIZE;
KASSERT(xbd_io->xio_start_offset < PAGE_SIZE);
KASSERT(bcount + xbd_io->xio_start_offset <= VBD_VA_SIZE);
(xbd_io->xio_vaddr + xbd_io->xio_start_offset);
vaddr_t xio_start_offset; /* I/O start offset */
#define xio_start_offset u.xio_rw.xio_start_offset