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

Eric Farman
2022-12-19T20:27:39Z

On Mon, 2022-12-19 at 14:27 -0500, Matthew Rosato wrote:
> 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?

Erp, yeah. I think I had that in one point; I'll work something in
here. Thanks for the top.

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