Re: [PATCH 1/1] vfio-ccw: Enable transparent CCW IPL from DASD

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 20 Apr 2020 14:26:17 +0200
Cornelia Huck <cohuck@redhat.com> wrote:

> On Fri, 17 Apr 2020 14:38:38 -0400
> Jared Rossi <jrossi@linux.ibm.com> wrote:
> 
> > Remove the explicit prefetch check when using vfio-ccw devices.
> > This check is not needed as all Linux channel programs are intended
> > to use prefetch and will be executed in the same way regardless.  
> 
> As already commented on the Linux patch: Can we log something, so this
> is debuggable if this statement does not hold true in the future?
> 
> > 
> > Signed-off-by: Jared Rossi <jrossi@linux.ibm.com>
> > ---
> >  hw/vfio/ccw.c | 13 +++----------
> >  1 file changed, 3 insertions(+), 10 deletions(-)
> > 
> > diff --git a/hw/vfio/ccw.c b/hw/vfio/ccw.c
> > index 50cc2ec75c..e649377b68 100644
> > --- a/hw/vfio/ccw.c
> > +++ b/hw/vfio/ccw.c
> > @@ -74,16 +74,9 @@ static IOInstEnding vfio_ccw_handle_request(SubchDev *sch)
> >      struct ccw_io_region *region = vcdev->io_region;
> >      int ret;
> >  
> > -    if (!(sch->orb.ctrl0 & ORB_CTRL0_MASK_PFCH)) {
> > -        if (!(vcdev->force_orb_pfch)) {
> > -            warn_once_pfch(vcdev, sch, "requires PFCH flag set");
> > -            sch_gen_unit_exception(sch);
> > -            css_inject_io_interrupt(sch);
> > -            return IOINST_CC_EXPECTED;
> > -        } else {
> > -            sch->orb.ctrl0 |= ORB_CTRL0_MASK_PFCH;
> > -            warn_once_pfch(vcdev, sch, "PFCH flag forced");
> > -        }
> > +    if (!(sch->orb.ctrl0 & ORB_CTRL0_MASK_PFCH) && vcdev->force_orb_pfch) {
> > +        sch->orb.ctrl0 |= ORB_CTRL0_MASK_PFCH;
> > +        warn_once_pfch(vcdev, sch, "PFCH flag forced");
> >      }  
> 
> What happens when you run it with an old kernel? I guess the I/O is
> only rejected later (after a trip into the kernel), but has that path
> ever been tested?
> 
> >  
> >      QEMU_BUILD_BUG_ON(sizeof(region->orb_area) != sizeof(ORB));  
> 

Oh, and do we want to deprecate the force prefetch interface in the
future? We probably need to wait a bit, until the kernel changes have
become widely available.