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

[Jared Rossi]

On 2020-04-20 08:29, Cornelia Huck wrote:
> On Mon, 20 Apr 2020 14:26:17 +0200
> Cornelia Huck <address@hidden> wrote:
>
> > On Fri, 17 Apr 2020 14:38:38 -0400
> > Jared Rossi <address@hidden> 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?

Agreed.  I will work on debugging improvements so that any future issues
related to unintended prefetching are more clearly logged.

> > >
> > > Signed-off-by: Jared Rossi <address@hidden>
> > > ---
> > >  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?

Yes, this was tested and you are correct that the kernel will reject the 
I/O unless
the corresponding patch is also applied there.  I will revisit this path 
while I'm
updating the logging to ensure that any potential interactions are 
appropriately
considered.

> > >
> > >      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.

Yes, I think we will want to deprecate it at an appropriate time in the 
future.