[Qemu-devel] [PATCH v2 1/2] vfio-ccw: add force unlimited prefetch property

[Halil Pasic]

There is at least one guest (OS) such 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, it fails to tell this to the machine.

Usually this ain't a big deal, as the original purpose of the P bit is to
allow for performance optimizations. vfio-ccw however can not provide the
guarantees required if the bit is not set.

It is impossible to implement support for P bit not set (at impossible
least without transitioning to lower level protocols) for vfio-ccw. So
let's give the user the opportunity to force the P bit to set, if the
user knows this is safe.  For self modifying channel programs forcing the
P bit is not safe. If P bit is forced for a self modifying channel
program things are expected to break in strange ways.

Signed-off-by: Halil Pasic <address@hidden>
Suggested-by: Dong Jia Shi <address@hidden>
Acked-by: Jason J. Herne <address@hidden>
Tested-by: Jason J. Herne <address@hidden>

---
v1 -> v2:
* reworded commit message
* re-factored the code (Connie)
* added warning when the P bit is forced the first time (Connie)

---
 hw/s390x/css.c |  3 +--
 hw/vfio/ccw.c  | 25 +++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/hw/s390x/css.c b/hw/s390x/css.c
index 56c3fa8c89..39ae5bbf7e 100644
--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -1204,8 +1204,7 @@ 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..62de4c9710 100644
--- a/hw/vfio/ccw.c
+++ b/hw/vfio/ccw.c
@@ -32,8 +32,20 @@ typedef struct VFIOCCWDevice {
     uint64_t io_region_offset;
     struct ccw_io_region *io_region;
     EventNotifier io_notifier;
+    bool force_orb_pfch;
+    bool warned_force_orb_pfch;
 } VFIOCCWDevice;
 
+#define WARN_ONCE(warned, fmt...) \
+({\
+if (!(warned)) {\
+    warn_report((fmt));\
+} \
+warned = true;\
+})
+
+
+
 static void vfio_ccw_compute_needs_reset(VFIODevice *vdev)
 {
     vdev->needs_reset = false;
@@ -54,6 +66,18 @@ 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_report("vfio-ccw 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(vcdev->warned_force_orb_pfch, "PFCH flag forced");
+        }
+    }
+
     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 +453,7 @@ static void vfio_ccw_unrealize(DeviceState *dev, Error 
**errp)
 
 static Property vfio_ccw_properties[] = {
     DEFINE_PROP_STRING("sysfsdev", VFIOCCWDevice, vdev.sysfsdev),
+    DEFINE_PROP_BOOL("force-orb-pfch", VFIOCCWDevice, force_orb_pfch, false),
     DEFINE_PROP_END_OF_LIST(),
 };
 
-- 
2.16.3