Re: [PATCH v1 11/16] vfio/ccw: discard second fmt-1 IDAW

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 11/21/22 4:40 PM, Eric Farman wrote:
> The intention is to read the first IDAW to determine the starting
> location of an I/O operation, knowing that the second and any/all
> subsequent IDAWs will be aligned per architecture. But, this read
> receives 64-bits of data, which is the size of a format-2 IDAW.
> 
> In the event that Format-1 IDAWs are presented, discard the lower
> 32 bits as they contain the second IDAW in such a list.
> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
>  drivers/s390/cio/vfio_ccw_cp.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/s390/cio/vfio_ccw_cp.c b/drivers/s390/cio/vfio_ccw_cp.c
> index 34a133d962d1..53246f4f95f7 100644
> --- a/drivers/s390/cio/vfio_ccw_cp.c
> +++ b/drivers/s390/cio/vfio_ccw_cp.c
> @@ -516,11 +516,15 @@ static int ccw_count_idaws(struct ccw1 *ccw,
>  		bytes = ccw->count;
>  
>  	if (ccw_is_idal(ccw)) {
> -		/* Read first IDAW to see if it's 4K-aligned or not. */
> -		/* All subsequent IDAws will be 4K-aligned. */
> +		/* Read first IDAW to check its starting address. */
> +		/* All subsequent IDAWs will be 2K- or 4K-aligned. */
>  		ret = vfio_dma_rw(vdev, ccw->cda, &iova, sizeof(iova), false);
>  		if (ret)
>  			return ret;
> +
> +		/* Format-1 IDAWs only occupy the first int */
> +		if (!cp->orb.cmd.c64)
> +			iova = iova >> 32;

Rather than read 8B and discarding 4B, can't we check this format value first and only read 4B for format-1?

>  	} else {
>  		iova = ccw->cda;
>  	}