[PATCH v2] s390x/css: revert SCSW ctrl/flag bits on error

[Peter Jin <pjin@linux.ibm.com>]

Revert the control and flag bits in the subchannel status word in case
the SSCH operation fails with non-zero CC (ditto for CSCH and HSCH).
According to POPS, the control and flag bits are only changed if SSCH,
CSCH, and HSCH return CC 0, and no other action should be taken otherwise.
In order to simulate that after the fact, the bits need to be reverted on
non-zero CC.

This change is necessary due to the fact that the pwrite() in vfio-ccw
which triggers the SSCH can fail at any time. Previously, there was
only virtio-ccw, whose do_subchannel_work function was only able to
return CC0. However, once vfio-ccw went into the mix, it has become
necessary to handle errors in code paths that were previously assumed
to always return success.

In our case, we found that in case of pwrite() failure (which was
discovered by strace injection), the subchannel could be stuck in start
pending state, which could be problematic if the pwrite() call returns
CC2. Experimentation shows that the guest tries to retry the SSCH call as
normal for CC2, but it actually continously fails due to the fact that
the subchannel is stuck in start pending state even though no start
function is actually taking place.

Signed-off-by: Peter Jin <pjin@linux.ibm.com>
---
 hw/s390x/css.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 48 insertions(+), 3 deletions(-)

diff --git a/hw/s390x/css.c b/hw/s390x/css.c
index 7d9523f811..95d1b3a3ce 100644
--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -1522,21 +1522,37 @@ IOInstEnding css_do_xsch(SubchDev *sch)
 IOInstEnding css_do_csch(SubchDev *sch)
 {
     SCHIB *schib = &sch->curr_status;
+    uint16_t old_scsw_ctrl;
+    IOInstEnding ccode;
 
     if (~(schib->pmcw.flags) & (PMCW_FLAGS_MASK_DNV | PMCW_FLAGS_MASK_ENA)) {
         return IOINST_CC_NOT_OPERATIONAL;
     }
 
+    /*
+     * Save the current scsw.ctrl in case CSCH fails and we need
+     * to revert the scsw to the status quo ante.
+     */
+    old_scsw_ctrl = schib->scsw.ctrl;
+
     /* Trigger the clear function. */
     schib->scsw.ctrl &= ~(SCSW_CTRL_MASK_FCTL | SCSW_CTRL_MASK_ACTL);
     schib->scsw.ctrl |= SCSW_FCTL_CLEAR_FUNC | SCSW_ACTL_CLEAR_PEND;
 
-    return do_subchannel_work(sch);
+    ccode = do_subchannel_work(sch);
+
+    if (ccode != IOINST_CC_EXPECTED) {
+        schib->scsw.ctrl = old_scsw_ctrl;
+    }
+
+    return ccode;
 }
 
 IOInstEnding css_do_hsch(SubchDev *sch)
 {
     SCHIB *schib = &sch->curr_status;
+    uint16_t old_scsw_ctrl;
+    IOInstEnding ccode;
 
     if (~(schib->pmcw.flags) & (PMCW_FLAGS_MASK_DNV | PMCW_FLAGS_MASK_ENA)) {
         return IOINST_CC_NOT_OPERATIONAL;
@@ -1553,6 +1569,12 @@ IOInstEnding css_do_hsch(SubchDev *sch)
         return IOINST_CC_BUSY;
     }
 
+    /*
+     * Save the current scsw.ctrl in case HSCH fails and we need
+     * to revert the scsw to the status quo ante.
+     */
+    old_scsw_ctrl = schib->scsw.ctrl;
+
     /* Trigger the halt function. */
     schib->scsw.ctrl |= SCSW_FCTL_HALT_FUNC;
     schib->scsw.ctrl &= ~SCSW_FCTL_START_FUNC;
@@ -1564,7 +1586,13 @@ IOInstEnding css_do_hsch(SubchDev *sch)
     }
     schib->scsw.ctrl |= SCSW_ACTL_HALT_PEND;
 
-    return do_subchannel_work(sch);
+    ccode = do_subchannel_work(sch);
+
+    if (ccode != IOINST_CC_EXPECTED) {
+        schib->scsw.ctrl = old_scsw_ctrl;
+    }
+
+    return ccode;
 }
 
 static void css_update_chnmon(SubchDev *sch)
@@ -1605,6 +1633,8 @@ static void css_update_chnmon(SubchDev *sch)
 IOInstEnding css_do_ssch(SubchDev *sch, ORB *orb)
 {
     SCHIB *schib = &sch->curr_status;
+    uint16_t old_scsw_ctrl, old_scsw_flags;
+    IOInstEnding ccode;
 
     if (~(schib->pmcw.flags) & (PMCW_FLAGS_MASK_DNV | PMCW_FLAGS_MASK_ENA)) {
         return IOINST_CC_NOT_OPERATIONAL;
@@ -1626,11 +1656,26 @@ IOInstEnding css_do_ssch(SubchDev *sch, ORB *orb)
     }
     sch->orb = *orb;
     sch->channel_prog = orb->cpa;
+
+    /*
+     * Save the current scsw.ctrl and scsw.flags in case SSCH fails and we need
+     * to revert the scsw to the status quo ante.
+     */
+    old_scsw_ctrl = schib->scsw.ctrl;
+    old_scsw_flags = schib->scsw.flags;
+
     /* Trigger the start function. */
     schib->scsw.ctrl |= (SCSW_FCTL_START_FUNC | SCSW_ACTL_START_PEND);
     schib->scsw.flags &= ~SCSW_FLAGS_MASK_PNO;
 
-    return do_subchannel_work(sch);
+    ccode = do_subchannel_work(sch);
+
+    if (ccode != IOINST_CC_EXPECTED) {
+        schib->scsw.ctrl = old_scsw_ctrl;
+        schib->scsw.flags = old_scsw_flags;
+    }
+
+    return ccode;
 }
 
 static void copy_irb_to_guest(IRB *dest, const IRB *src, const PMCW *pmcw,
-- 
2.37.3