Re: [PATCH v1 10/16] vfio/ccw: refactor the idaw counter

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

On 11/21/22 4:40 PM, Eric Farman wrote:
> The rules of an IDAW are fairly simple: Each one can move no
> more than a defined amount of data, must not cross the
> boundary defined by that length, and must be aligned to that
> length as well. The first IDAW in a list is special, in that
> it does not need to adhere to that alignment, but the other
> rules still apply. Thus, by reading the first IDAW in a list,
> the number of IDAWs that will comprise a data transfer of a
> particular size can be calculated.
> 
> Let's factor out the reading of that first IDAW with the
> logic that calculates the length of the list, to simplify
> the rest of the routine that handles the individual IDAWs.
> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
>  drivers/s390/cio/vfio_ccw_cp.c | 39 ++++++++++++++++++++++++++--------
>  1 file changed, 30 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/s390/cio/vfio_ccw_cp.c b/drivers/s390/cio/vfio_ccw_cp.c
> index a30f26962750..34a133d962d1 100644
> --- a/drivers/s390/cio/vfio_ccw_cp.c
> +++ b/drivers/s390/cio/vfio_ccw_cp.c
> @@ -496,23 +496,25 @@ static int ccwchain_fetch_tic(struct ccw1 *ccw,
>  	return -EFAULT;
>  }
>  
> -static int ccwchain_fetch_ccw(struct ccw1 *ccw,
> -			      struct page_array *pa,
> -			      struct channel_program *cp)
> +/*
> + * ccw_count_idaws() - Calculate the number of IDAWs needed to transfer
> + * a specified amount of data
> + *
> + * @ccw: The Channel Command Word being translated
> + * @cp: Channel Program being processed
> + */
> +static int ccw_count_idaws(struct ccw1 *ccw,
> +			   struct channel_program *cp)
>  {
>  	struct vfio_device *vdev =
>  		&container_of(cp, struct vfio_ccw_private, cp)->vdev;
>  	u64 iova;
> -	unsigned long *idaws;
>  	int ret;
>  	int bytes = 1;
> -	int idaw_nr, idal_len;
> -	int i;
>  
>  	if (ccw->count)
>  		bytes = ccw->count;
>  
> -	/* Calculate size of IDAL */
>  	if (ccw_is_idal(ccw)) {
>  		/* Read first IDAW to see if it's 4K-aligned or not. */
>  		/* All subsequent IDAws will be 4K-aligned. */
> @@ -522,7 +524,26 @@ static int ccwchain_fetch_ccw(struct ccw1 *ccw,
>  	} else {
>  		iova = ccw->cda;
>  	}
> -	idaw_nr = idal_nr_words((void *)iova, bytes);
> +
> +	return idal_nr_words((void *)iova, bytes);
> +}
> +
> +static int ccwchain_fetch_ccw(struct ccw1 *ccw,
> +			      struct page_array *pa,
> +			      struct channel_program *cp)
> +{
> +	struct vfio_device *vdev =
> +		&container_of(cp, struct vfio_ccw_private, cp)->vdev;
> +	unsigned long *idaws;
> +	int ret;
> +	int idaw_nr, idal_len;
> +	int i;
> +
> +	/* Calculate size of IDAL */
> +	idaw_nr = ccw_count_idaws(ccw, cp);
> +	if (idaw_nr < 0)
> +		return idaw_nr;
> +

What about if we get a 0 back from ccw_count_idaws?   The next thing we're going to do (not shown here) is kcalloc(0, sizeof(*idaws)), which I think means you'll get back ZERO_SIZE_PTR, not a null pointer.

>  	idal_len = idaw_nr * sizeof(*idaws);
>  
>  	/* Allocate an IDAL from host storage */
> @@ -555,7 +576,7 @@ static int ccwchain_fetch_ccw(struct ccw1 *ccw,
>  		for (i = 0; i < idaw_nr; i++)
>  			pa->pa_iova[i] = idaws[i];
>  	} else {
> -		pa->pa_iova[0] = iova;
> +		pa->pa_iova[0] = ccw->cda;
>  		for (i = 1; i < pa->pa_nr; i++)
>  			pa->pa_iova[i] = pa->pa_iova[i - 1] + PAGE_SIZE;
>  	}