[PATCH v2 4/6] spapr/xive: Enforce load-after-store ordering

["Cédric Le Goater" <clg@kaod.org>]

The XIVE_ESB_SET_PQ_10 load operation is used to disable temporarily
an interrupt source. If StoreEOI is active, a source could be left
enabled if the load and store operations come out of order.

QEMU makes use of this offset to quiesce the sources before a
migration. Enforce the load-after-store ordering always when doing so
without querying the characteristics of the sources on the host. The
performance penalty will be very small for QEMU.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 include/hw/ppc/xive.h    |  8 ++++++++
 hw/intc/spapr_xive_kvm.c | 12 ++++++++++++
 hw/intc/xive.c           |  6 ++++++
 3 files changed, 26 insertions(+)

diff --git a/include/hw/ppc/xive.h b/include/hw/ppc/xive.h
index 445eccfe6b73..39cd273f86d5 100644
--- a/include/hw/ppc/xive.h
+++ b/include/hw/ppc/xive.h
@@ -279,6 +279,14 @@ static inline hwaddr xive_source_esb_mgmt(XiveSource *xsrc, int srcno)
 #define XIVE_ESB_SET_PQ_10      0xe00 /* Load */
 #define XIVE_ESB_SET_PQ_11      0xf00 /* Load */
 
+/*
+ * Load-after-store ordering
+ *
+ * Adding this offset to the load address will enforce
+ * load-after-store ordering. This is required to use with StoreEOI.
+ */
+#define XIVE_ESB_LD_ST_MO       0x40 /* Load-after-store ordering */
+
 uint8_t xive_source_esb_get(XiveSource *xsrc, uint32_t srcno);
 uint8_t xive_source_esb_set(XiveSource *xsrc, uint32_t srcno, uint8_t pq);
 
diff --git a/hw/intc/spapr_xive_kvm.c b/hw/intc/spapr_xive_kvm.c
index 66bf4c06fe55..d428422a7b72 100644
--- a/hw/intc/spapr_xive_kvm.c
+++ b/hw/intc/spapr_xive_kvm.c
@@ -357,6 +357,18 @@ static uint64_t xive_esb_rw(XiveSource *xsrc, int srcno, uint32_t offset,
 
 static uint8_t xive_esb_read(XiveSource *xsrc, int srcno, uint32_t offset)
 {
+    /*
+     * The XIVE_ESB_SET_PQ_10 load operation is used to disable
+     * temporarily an interrupt source. If StoreEOI is active, a
+     * source could be left enabled if the load and store operations
+     * come out of order.
+     *
+     * Enforce the load-after-store ordering always.
+     */
+    if (offset == XIVE_ESB_SET_PQ_10) {
+        offset |= XIVE_ESB_LD_ST_MO;
+    }
+
     return xive_esb_rw(xsrc, srcno, offset, 0, 0) & 0x3;
 }
 
diff --git a/hw/intc/xive.c b/hw/intc/xive.c
index 489e6256ef70..b710ba2df095 100644
--- a/hw/intc/xive.c
+++ b/hw/intc/xive.c
@@ -998,6 +998,12 @@ static uint64_t xive_source_esb_read(void *opaque, hwaddr addr, unsigned size)
     case XIVE_ESB_SET_PQ_01 ... XIVE_ESB_SET_PQ_01 + 0x0FF:
     case XIVE_ESB_SET_PQ_10 ... XIVE_ESB_SET_PQ_10 + 0x0FF:
     case XIVE_ESB_SET_PQ_11 ... XIVE_ESB_SET_PQ_11 + 0x0FF:
+        if (offset == XIVE_ESB_SET_PQ_10 &&
+            xsrc->esb_flags & XIVE_SRC_STORE_EOI) {
+            qemu_log_mask(LOG_GUEST_ERROR, "XIVE: load-after-store ordering "
+                          "not enforced with Store EOI active for IRQ %d\n",
+                          srcno);
+        }
         ret = xive_source_esb_set(xsrc, srcno, (offset >> 8) & 0x3);
         break;
     default:
-- 
2.25.4