kvm: error out of kvm_irqchip_add_msi_route() in case of full route table

subj is calling kvm_add_routing_entry() which simply extends
  KVMState::irq_routes::entries[]
but doesn't check if number of routes goes beyond limit the kernel
is willing to accept. Which later leads toi the assert

  qemu-kvm: ../accel/kvm/kvm-all.c:1833: kvm_irqchip_commit_routes: Assertion `ret == 0' failed

typically it happens during guest boot for large enough guest

Reproduced with:
  ./qemu --enable-kvm -m 8G -smp 64 -machine pc \
     `for b in {1..2}; do echo -n "-device pci-bridge,id=pci$b,chassis_nr=$b ";
        for i in {0..31}; do touch /tmp/vblk$b$i;
           echo -n "-drive file=/tmp/vblk$b$i,if=none,id=drive$b$i,format=raw
                    -device virtio-blk-pci,drive=drive$b$i,bus=pci$b ";
      done; done`

While crash at boot time is bad, the same might happen at hotplug time
which is unacceptable.
So instead calling kvm_add_routing_entry() unconditionally, check first
that number of routes won't exceed KVM_CAP_IRQ_ROUTING. This way virtio
device insteads killin qemu, will gracefully fail to initialize device
as expected with following warnings on console:
    virtio-blk failed to set guest notifier (-28), ensure -accel kvm is set.
    virtio_bus_start_ioeventfd: failed. Fallback to userspace (slower).

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-ID: <20240408110956.451558-1-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 10 insertions, 5 deletions

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index a8cecd040e..931f74256e 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -1999,12 +1999,17 @@ int kvm_irqchip_add_msi_route(KVMRouteChange *c, int vector, PCIDevice *dev)
         return -EINVAL;
     }
 
-    trace_kvm_irqchip_add_msi_route(dev ? dev->name : (char *)"N/A",
-                                    vector, virq);
+    if (s->irq_routes->nr < s->gsi_count) {
+        trace_kvm_irqchip_add_msi_route(dev ? dev->name : (char *)"N/A",
+                                        vector, virq);
 
-    kvm_add_routing_entry(s, &kroute);
-    kvm_arch_add_msi_route_post(&kroute, vector, dev);
-    c->changes++;
+        kvm_add_routing_entry(s, &kroute);
+        kvm_arch_add_msi_route_post(&kroute, vector, dev);
+        c->changes++;
+    } else {
+        kvm_irqchip_release_virq(s, virq);
+        return -ENOSPC;
+    }
 
     return virq;
 }