[Qemu-devel] [PATCH v4 12/28] ppc/xive: notify the CPU when the interrupt priority is more privileged

["Cédric Le Goater" <clg@kaod.org>]

After the event was pushed in the Xive EQ, the IVPE raises the bit
corresponding to the priority of the pending interrupt in the register
IBP (Interrupt Pending Buffer) to indicate there is an event pending
in one of the 8 priority queues. The Pending Interrupt Priority
Register (PIPR) is also updated using the IPB. This register represent
the priority of the most favored pending notification.

The PIPR is then compared to the the Current Processor Priority
Register (CPPR). If it is more favored (numerically less than), the
CPU interrupt line is raised and the EO bit of the Notification Source
Register (NSR) is updated to notify the presence of an exception for
the O/S. The check needs to be done whenever the PIPR or the CPPR are
changed.

The O/S acknowledges the interrupt with a special load in the Thread
Interrupt Management Area. If the EO bit of the NSR is set, the CPPR
takes the value of PIPR. The bit number in the IBP corresponding to
the priority of the pending interrupt is reseted and so is the EO bit
of the NSR.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 hw/intc/xive.c | 94 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 93 insertions(+), 1 deletion(-)

diff --git a/hw/intc/xive.c b/hw/intc/xive.c
index 6ea9441852e3..f249ffc8943e 100644
--- a/hw/intc/xive.c
+++ b/hw/intc/xive.c
@@ -21,9 +21,73 @@
  * XIVE Thread Interrupt Management context
  */
 
+/* Convert a priority number to an Interrupt Pending Buffer (IPB)
+ * register, which indicates a pending interrupt at the priority
+ * corresponding to the bit number
+ */
+static uint8_t priority_to_ipb(uint8_t priority)
+{
+    return priority > XIVE_PRIORITY_MAX ?
+        0 : 1 << (XIVE_PRIORITY_MAX - priority);
+}
+
+/* Convert an Interrupt Pending Buffer (IPB) register to a Pending
+ * Interrupt Priority Register (PIPR), which contains the priority of
+ * the most favored pending notification.
+ */
+static uint8_t ipb_to_pipr(uint8_t ibp)
+{
+    return ibp ? clz32((uint32_t)ibp << 24) : 0xff;
+}
+
+static void ipb_update(uint8_t *regs, uint8_t priority)
+{
+    regs[TM_IPB] |= priority_to_ipb(priority);
+    regs[TM_PIPR] = ipb_to_pipr(regs[TM_IPB]);
+}
+
+static uint8_t exception_mask(uint8_t ring)
+{
+    switch (ring) {
+    case TM_QW1_OS:
+        return TM_QW1_NSR_EO;
+    default:
+        g_assert_not_reached();
+    }
+}
+
 static uint64_t xive_tctx_accept(XiveTCTX *tctx, uint8_t ring)
 {
-    return 0;
+    uint8_t *regs = &tctx->regs[ring];
+    uint8_t nsr = regs[TM_NSR];
+    uint8_t mask = exception_mask(ring);
+
+    qemu_irq_lower(tctx->output);
+
+    if (regs[TM_NSR] & mask) {
+        uint8_t cppr = regs[TM_PIPR];
+
+        regs[TM_CPPR] = cppr;
+
+        /* Reset the pending buffer bit */
+        regs[TM_IPB] &= ~priority_to_ipb(cppr);
+        regs[TM_PIPR] = ipb_to_pipr(regs[TM_IPB]);
+
+        /* Drop Exception bit */
+        regs[TM_NSR] &= ~mask;
+    }
+
+    return (nsr << 8) | regs[TM_CPPR];
+}
+
+static void xive_tctx_notify(XiveTCTX *tctx, uint8_t ring)
+{
+    uint8_t *regs = &tctx->regs[ring];
+
+    if (regs[TM_PIPR] < regs[TM_CPPR]) {
+        regs[TM_NSR] |= exception_mask(ring);
+        qemu_irq_raise(tctx->output);
+    }
 }
 
 static void xive_tctx_set_cppr(XiveTCTX *tctx, uint8_t ring, uint8_t cppr)
@@ -33,6 +97,9 @@ static void xive_tctx_set_cppr(XiveTCTX *tctx, uint8_t ring, uint8_t cppr)
     }
 
     tctx->regs[ring + TM_CPPR] = cppr;
+
+    /* CPPR has changed, check if we need to raise a pending exception */
+    xive_tctx_notify(tctx, ring);
 }
 
 /*
@@ -199,6 +266,17 @@ static void xive_tm_set_os_cppr(XiveTCTX *tctx, hwaddr offset,
 }
 
 /*
+ * Adjust the IPB to allow a CPU to process event queues of other
+ * priorities during one physical interrupt cycle.
+ */
+static void xive_tm_set_os_pending(XiveTCTX *tctx, hwaddr offset,
+                                   uint64_t value, unsigned size)
+{
+    ipb_update(&tctx->regs[TM_QW1_OS], value & 0xff);
+    xive_tctx_notify(tctx, TM_QW1_OS);
+}
+
+/*
  * Define a mapping of "special" operations depending on the TIMA page
  * offset and the size of the operation.
  */
@@ -220,6 +298,7 @@ static const XiveTmOp xive_tm_operations[] = {
 
     /* MMIOs above 2K : special operations with side effects */
     { XIVE_TM_OS_PAGE, TM_SPC_ACK_OS_REG,     2, NULL, xive_tm_ack_os_reg },
+    { XIVE_TM_OS_PAGE, TM_SPC_SET_OS_PENDING, 1, xive_tm_set_os_pending, NULL },
 };
 
 static const XiveTmOp *xive_tm_find_op(hwaddr offset, unsigned size, bool write)
@@ -414,6 +493,13 @@ static void xive_tctx_reset(void *dev)
     tctx->regs[TM_QW1_OS + TM_LSMFB] = 0xFF;
     tctx->regs[TM_QW1_OS + TM_ACK_CNT] = 0xFF;
     tctx->regs[TM_QW1_OS + TM_AGE] = 0xFF;
+
+    /*
+     * Initialize PIPR to 0xFF to avoid phantom interrupts when the
+     * CPPR is first set.
+     */
+    tctx->regs[TM_QW1_OS + TM_PIPR] =
+        ipb_to_pipr(tctx->regs[TM_QW1_OS + TM_IPB]);
 }
 
 static void xive_tctx_realize(DeviceState *dev, Error **errp)
@@ -1185,9 +1271,15 @@ static void xive_presenter_notify(XiveRouter *xrtr, uint8_t format,
     found = xive_presenter_match(xrtr, format, vp_blk, vp_idx, cam_ignore,
                                  priority, logic_serv, &match);
     if (found) {
+        ipb_update(&match.tctx->regs[match.ring], priority);
+        xive_tctx_notify(match.tctx, match.ring);
         return;
     }
 
+    /* Record the IPB in the associated VP */
+    ipb_update((uint8_t *) &vp.w4, priority);
+    xive_router_set_vp(xrtr, vp_blk, vp_idx, &vp);
+
     /* If no VP dispatched on a HW thread :
      * - update the VP if backlog is activated
      * - escalate (ESe PQ bits and IVE in w4-5) if escalation is
-- 
2.13.6