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

Matthew Rosato
2022-12-19T19:41:00Z

On 12/19/22 2:31 PM, Eric Farman wrote:
> On Mon, 2022-12-19 at 14:16 -0500, Matthew Rosato wrote:
>> 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.
>
> While it's true that the idal_nr_words routines could return zero, I
> don't see how the ccw_count_idaws routine which calls it could do the
> same. We added a check for a zero data count with commit 453eac312445e
> ("s390/cio: Allow zero-length CCWs in vfio-ccw"), such that a CCW that
> has no length will cause us to allocate -something- that would be valid
> for the channel to use, even if it's not going to put anything in/out
> of it.

Ah, yeah I was specifically looking at idal_nr_words and I missed the 'int bytes = 1;' business at the top of ccw_count_idaws.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>

>
>>
>>>         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;
>>>         }
>>
>