[Qemu-devel] [PATCH 1/2] vfio-ccw: add force unlimited prefetch property

[Halil Pasic <pasic@linux.ibm.com>]

There is at least one control program (guest) that although it does not
rely on the guarantees provided by ORB 1 word 9 bit (aka unlimited
prefetch, aka P bit) not being set, fails to tell this to the machine.

Usually this ain't a big deal, as the story is usually about performance
optimizations only. But vfio-ccw can not provide the guarantees required
if the bit is not set.

Since it is impossible to implement support for P bit not set (at
impossible least without transitioning to lower level protocols) in
vfio-ccw let's provide a manual override.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Suggested-by: Dong Jia Shi <bjsdjshi@linux.ibm.com>
Acked-by: Jason J. Herne <jjherne@linux.ibm.com>
Tested-by: Jason J. Herne <jjherne@linux.ibm.com>
---
 hw/s390x/css.c |  3 +--
 hw/vfio/ccw.c  | 13 +++++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/hw/s390x/css.c b/hw/s390x/css.c
index 301bf1772f..32f1b2820d 100644
--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -1196,8 +1196,7 @@ static IOInstEnding sch_handle_start_func_passthrough(SubchDev *sch)
      * Only support prefetch enable mode.
      * Only support 64bit addressing idal.
      */
-    if (!(orb->ctrl0 & ORB_CTRL0_MASK_PFCH) ||
-        !(orb->ctrl0 & ORB_CTRL0_MASK_C64)) {
+    if (!(orb->ctrl0 & ORB_CTRL0_MASK_C64)) {
         warn_report("vfio-ccw requires PFCH and C64 flags set");
         sch_gen_unit_exception(sch);
         css_inject_io_interrupt(sch);
diff --git a/hw/vfio/ccw.c b/hw/vfio/ccw.c
index e67392c5f9..32cf606a71 100644
--- a/hw/vfio/ccw.c
+++ b/hw/vfio/ccw.c
@@ -32,6 +32,8 @@ typedef struct VFIOCCWDevice {
     uint64_t io_region_offset;
     struct ccw_io_region *io_region;
     EventNotifier io_notifier;
+    /* force unlimited prefetch */
+    bool f_upfch;
 } VFIOCCWDevice;
 
 static void vfio_ccw_compute_needs_reset(VFIODevice *vdev)
@@ -52,8 +54,18 @@ static IOInstEnding vfio_ccw_handle_request(SubchDev *sch)
     S390CCWDevice *cdev = sch->driver_data;
     VFIOCCWDevice *vcdev = DO_UPCAST(VFIOCCWDevice, cdev, cdev);
     struct ccw_io_region *region = vcdev->io_region;
+    bool upfch = sch->orb.ctrl0 & ORB_CTRL0_MASK_PFCH;
     int ret;
 
+    if (!upfch && !vcdev->f_upfch) {
+        warn_report("vfio-ccw requires PFCH flag set");
+        sch_gen_unit_exception(sch);
+        css_inject_io_interrupt(sch);
+        return IOINST_CC_EXPECTED;
+    } else if (!upfch) {
+        sch->orb.ctrl0 |= ORB_CTRL0_MASK_PFCH;
+    }
+
     QEMU_BUILD_BUG_ON(sizeof(region->orb_area) != sizeof(ORB));
     QEMU_BUILD_BUG_ON(sizeof(region->scsw_area) != sizeof(SCSW));
     QEMU_BUILD_BUG_ON(sizeof(region->irb_area) != sizeof(IRB));
@@ -429,6 +441,7 @@ static void vfio_ccw_unrealize(DeviceState *dev, Error **errp)
 
 static Property vfio_ccw_properties[] = {
     DEFINE_PROP_STRING("sysfsdev", VFIOCCWDevice, vdev.sysfsdev),
+    DEFINE_PROP_BOOL("f-upfch", VFIOCCWDevice, f_upfch, false),
     DEFINE_PROP_END_OF_LIST(),
 };
 
-- 
2.16.3