[PATCH 5/8] spapr/xive: Enforce the load-after-store ordering

[Cédric Le Goater]

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 ++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/include/hw/ppc/xive.h b/include/hw/ppc/xive.h
index 2c42ae92d287..c061230ea802 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 90f4509e6959..3eea4cb1c49f 100644
--- a/hw/intc/spapr_xive_kvm.c
+++ b/hw/intc/spapr_xive_kvm.c
@@ -317,6 +317,18 @@ 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;
 }
 
-- 
2.25.4