[PATCH v2] s390x/css: Selectively copy sense data to IRB

[Eric Farman <farman@linux.ibm.com>]

The SCHIB.PMCW.CSENSE bit is used to determine whether the
IRB should be set up with sense data, but that bit only
indicates whether sense data is requested, not if it was
provided by the device. For virtual devices, this is fine.

For passthrough devices, hardware would present sense data
in IRB.ECW, but that field is only valid if IRB.SCSW.E and
IRB.ERW.S were also set.

Let's only build the sense data in the IRB if the first byte
of sense is nonzero (indicating it may have come from a virtual
device), or the IRB.SCSW.E bit is already set (indicating it
came from the hardware). That way, the guest driver can read
the sense data if valid, or respond with a Sense CCW to get
the sense data if it wants/needs.

Fixes: df1fe5bb4924 ("s390: Virtual channel subsystem support.")
Fixes: 334e76850bbb ("vfio/ccw: update sense data if a unit check is pending")
Signed-off-by: Eric Farman <farman@linux.ibm.com>
---

Notes:
    v1->v2:
     - [MR] Add Fixes: tags
     - [CH] Reinstate the memcpy(sch->sense_data, irb.ecw) in vfio_ccw
     - [CH] Look at IRB.SCSW.E before copying sense into guest IRB
    
    v1: https://lore.kernel.org/qemu-devel/20210610202011.391029-1-farman@linux.ibm.com/

 hw/s390x/css.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/hw/s390x/css.c b/hw/s390x/css.c
index bed46f5ec3..8935f948d5 100644
--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -1659,9 +1659,15 @@ int css_do_tsch_get_irb(SubchDev *sch, IRB *target_irb, int *irb_len)
         } else {
             irb.esw[0] = 0x00800000;
         }
-        /* If a unit check is pending, copy sense data. */
+        /*
+         * If a unit check is pending and concurrent sense
+         * is requested, copy the sense data if the sense
+         * data is plausibly valid.
+         */
         if ((schib->scsw.dstat & SCSW_DSTAT_UNIT_CHECK) &&
-            (schib->pmcw.chars & PMCW_CHARS_MASK_CSENSE)) {
+            (schib->pmcw.chars & PMCW_CHARS_MASK_CSENSE) &&
+            ((schib->scsw.flags & SCSW_FLAGS_MASK_ECTL) ||
+             (sch->sense_data[0] != 0))) {
             int i;
 
             irb.scsw.flags |= SCSW_FLAGS_MASK_ESWF | SCSW_FLAGS_MASK_ECTL;
-- 
2.25.1