[Qemu-devel] [PATCH v2 2/2] spapr/irq: Only claim VALID interrupts at the KVM level

["Cédric Le Goater" <clg@kaod.org>]

A typical pseries VM with 16 vCPUs, one disk, one network adapater
uses less than 100 interrupts but the whole IRQ number space of the
QEMU machine is allocated at reset time and it is 8K wide. This is
wasting a considerable amount of interrupt numbers in the global IRQ
space which has 1M interrupts per socket on a POWER9.

To optimise the HW resources, only request at the KVM level interrupts
which have been claimed by the guest. This will help to increase the
maximum number of VMs per system and also help supporting nested guests
using the XIVE interrupt mode.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
---
 hw/intc/spapr_xive_kvm.c | 29 ++++++++++++++++++++++++++---
 hw/intc/xics_kvm.c       |  8 ++++++++
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/hw/intc/spapr_xive_kvm.c b/hw/intc/spapr_xive_kvm.c
index 17af4d19f54e..71b88d7797bc 100644
--- a/hw/intc/spapr_xive_kvm.c
+++ b/hw/intc/spapr_xive_kvm.c
@@ -255,11 +255,16 @@ void kvmppc_xive_source_reset_one(XiveSource *xsrc, int srcno, Error **errp)
 
 static void kvmppc_xive_source_reset(XiveSource *xsrc, Error **errp)
 {
+    SpaprXive *xive = SPAPR_XIVE(xsrc->xive);
     int i;
 
     for (i = 0; i < xsrc->nr_irqs; i++) {
         Error *local_err = NULL;
 
+        if (!xive_eas_is_valid(&xive->eat[i])) {
+            continue;
+        }
+
         kvmppc_xive_source_reset_one(xsrc, i, &local_err);
         if (local_err) {
             error_propagate(errp, local_err);
@@ -328,11 +333,18 @@ uint64_t kvmppc_xive_esb_rw(XiveSource *xsrc, int srcno, uint32_t offset,
 
 static void kvmppc_xive_source_get_state(XiveSource *xsrc)
 {
+    SpaprXive *xive = SPAPR_XIVE(xsrc->xive);
     int i;
 
     for (i = 0; i < xsrc->nr_irqs; i++) {
+        uint8_t pq;
+
+        if (!xive_eas_is_valid(&xive->eat[i])) {
+            continue;
+        }
+
         /* Perform a load without side effect to retrieve the PQ bits */
-        uint8_t pq = xive_esb_read(xsrc, i, XIVE_ESB_GET);
+        pq = xive_esb_read(xsrc, i, XIVE_ESB_GET);
 
         /* and save PQ locally */
         xive_source_esb_set(xsrc, i, pq);
@@ -521,9 +533,14 @@ static void kvmppc_xive_change_state_handler(void *opaque, int running,
      */
     if (running) {
         for (i = 0; i < xsrc->nr_irqs; i++) {
-            uint8_t pq = xive_source_esb_get(xsrc, i);
+            uint8_t pq;
             uint8_t old_pq;
 
+            if (!xive_eas_is_valid(&xive->eat[i])) {
+                continue;
+            }
+
+            pq = xive_source_esb_get(xsrc, i);
             old_pq = xive_esb_read(xsrc, i, XIVE_ESB_SET_PQ_00 + (pq << 8));
 
             /*
@@ -545,7 +562,13 @@ static void kvmppc_xive_change_state_handler(void *opaque, int running,
      * migration is in progress.
      */
     for (i = 0; i < xsrc->nr_irqs; i++) {
-        uint8_t pq = xive_esb_read(xsrc, i, XIVE_ESB_GET);
+        uint8_t pq;
+
+        if (!xive_eas_is_valid(&xive->eat[i])) {
+            continue;
+        }
+
+        pq = xive_esb_read(xsrc, i, XIVE_ESB_GET);
 
         /*
          * PQ is set to PENDING to possibly catch a triggered
diff --git a/hw/intc/xics_kvm.c b/hw/intc/xics_kvm.c
index a4d2e876cc5f..ba90d6dc966c 100644
--- a/hw/intc/xics_kvm.c
+++ b/hw/intc/xics_kvm.c
@@ -190,6 +190,10 @@ void ics_get_kvm_state(ICSState *ics)
     for (i = 0; i < ics->nr_irqs; i++) {
         ICSIRQState *irq = &ics->irqs[i];
 
+        if (ics_irq_free(ics, i)) {
+            continue;
+        }
+
         kvm_device_access(kernel_xics_fd, KVM_DEV_XICS_GRP_SOURCES,
                           i + ics->offset, &state, false, &error_fatal);
 
@@ -301,6 +305,10 @@ int ics_set_kvm_state(ICSState *ics, Error **errp)
         Error *local_err = NULL;
         int ret;
 
+        if (ics_irq_free(ics, i)) {
+            continue;
+        }
+
         ret = ics_set_kvm_state_one(ics, i, &local_err);
         if (ret < 0) {
             error_propagate(errp, local_err);
-- 
2.21.0