Re: [PATCH 1/1] vfio-ccw: Keep passthrough sense data intact

[Eric Farman]

On Fri, 2021-06-11 at 12:21 +0200, Cornelia Huck wrote:
> On Fri, Jun 11 2021, Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > On Thu, Jun 10 2021, Eric Farman <farman@linux.ibm.com> wrote:
> > > diff --git a/hw/s390x/css.c b/hw/s390x/css.c
> > > index bed46f5ec3..29234daa27 100644
> > > --- a/hw/s390x/css.c
> > > +++ b/hw/s390x/css.c
> > > @@ -1661,7 +1661,8 @@ int css_do_tsch_get_irb(SubchDev *sch, IRB
> > > *target_irb, int *irb_len)
> > >          }
> > >          /* If a unit check is pending, copy sense data. */
> > >          if ((schib->scsw.dstat & SCSW_DSTAT_UNIT_CHECK) &&
> > > -            (schib->pmcw.chars & PMCW_CHARS_MASK_CSENSE)) {
> > > +            (schib->pmcw.chars & PMCW_CHARS_MASK_CSENSE) &&
> > > +            (sch->sense_data[0] != 0)) {
> > >              int i;
> > >  
> > >              irb.scsw.flags |= SCSW_FLAGS_MASK_ESWF |
> > >              SCSW_FLAGS_MASK_ECTL;
> 
> This function is where we build the esw/ecw...
> 
> > > diff --git a/hw/vfio/ccw.c b/hw/vfio/ccw.c
> > > index 139a3d9d1b..a4dc4acb34 100644
> > > --- a/hw/vfio/ccw.c
> > > +++ b/hw/vfio/ccw.c
> > > @@ -371,12 +371,6 @@ static void
> > > vfio_ccw_io_notifier_handler(void *opaque)
> > >      copy_scsw_to_guest(&s, &irb.scsw);
> > >      schib->scsw = s;
> > >  
> > > -    /* If a uint check is pending, copy sense data. */
> > > -    if ((schib->scsw.dstat & SCSW_DSTAT_UNIT_CHECK) &&
> > > -        (schib->pmcw.chars & PMCW_CHARS_MASK_CSENSE)) {
> 
> ...and here we actually do have the esw/ecw provided by the hardware.
> 
> > If I'm reading the PoP correctly, turning on concurrent sense only
> > means
> > that we may have sense data already available, but not that it's
> > guaranteed.

Agreed.

> >  Would it be enough to look at the relevant bit in the erw
> > and only copy sense data if it is actually set (here and/or above)?

Do we have the hardware ERW in the css_do_tsch routine?

Oh, but we have SCSW, and POPS says if ERW.S is set, SCSW.E is set. So
that would make this a pretty simple change then.

> 
> Maybe the root of the problem is that we actually try to build the
> esw
> ourselves? If we copy it from the irb received by the hardware, we
> should already have the correct data, I think.

Yeah, that's part of the problem. As you note above, the PMCW.CSENSE
bit only says if concurrent sense is possible, not that it was actually
stored in the IRB.

I (mistakenly) thought that removing this hunk would get the whole IRB
copied over, but I see now that css_do_tsch_get_irb() only copies the
SCSW, and builds the ESW/ECW based off sch->sense_data.

> 
> > > -        memcpy(sch->sense_data, irb.ecw, sizeof(irb.ecw));
> > > -    }
> > > -
> > >  read_err:
> > >      css_inject_io_interrupt(sch);
> > >  }