Fix irq route entries exceed KVM_MAX_IRQ_ROUTES

Last month, we experienced several guests crash(6cores-8cores),qemu logs display the following messages:

qemu-system-x86_64: /build/qemu-2.1.2/kvm-all.c:976:
kvm_irqchip_commit_routes: Assertion `ret == 0' failed.

After analysis and verification, we can confirm it's irq-balance daemon(in guest) leads to the assertion failure.So start a 8 core guest with two disks, execute the following scripts will reproduce the BUG quickly:

vda_irq_num=25
vdb_irq_num=27
while [ 1 ]
do
for irq in {1,2,4,8,10,20,40,80}
do
echo $irq > /proc/irq/$vda_irq_num/smp_affinity echo $irq > /proc/irq/$vdb_irq_num/smp_affinity dd if=/dev/vda of=/dev/zero bs=4K count=100 iflag=direct dd if=/dev/vdb of=/dev/zero bs=4K count=100 iflag=direct
done
done

QEMU setup static irq route entries in kvm_pc_setup_irq_routing(),PIC and IOAPIC share the first 15 GSI numbers,take up 23 GSI numbers,but take up 38 irq route entries.When change irq smp_affinity in guest,a dynamic route entry may be setup,the current logic is:if allocate GSI number succeeds, a new route entry can be added.The available dynamic GSI numbers is 1021(KVM_MAX_IRQ_ROUTES-23),but available irq route entries is only 986(KVM_MAX_IRQ_ROUTES-38),GSI numbers greater than route entries. irq-balance's behavior will eventually leads to total irq route entries exceed KVM_MAX_IRQ_ROUTES,ioctl(KVM_SET_GSI_ROUTING) fail and kvm_irqchip_commit_routes() trigger assertion failure.

This patch fix the BUG.

---
 kvm-all.c |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

index 18cc6b4..f47e1b1 100644
@@ -1123,6 +1123,17 @@ static int kvm_irqchip_get_virq(KVMState *s)
     int i, bit;
     bool retry = true;
 
+    /*
+     * PIC and IOAPIC share the first 15 GSI numbers,available GSI
+     * numbers greater than IRQ route entries. If allocate GSI number
+     * succeeds, a new route entry can be added, so total IRQ route
+     * enties can exceed gsi_count, flush dynamic MSI entries when
+     * IRQ route entries arrive gsi_count.
+     */
+    if (!s->direct_msi && s->irq_routes->nr == s->gsi_count) {
+        kvm_flush_dynamic_msi_routes(s);
+    }
+
 again:
     /* Return the lowest unused GSI in the bitmap */
     for (i = 0; i < max_words; i++) {

1.7.1

Ping

Patches here:
http://patchwork.ozlabs.org/patch/424738/

Description:
In multi-core guest, set irq affinity will eventually lead to guest crash, this is a severe BUG, I do not know why this patch was ignored?

Wenshuang Ma

Because there is one maintainer and he (I) was on holiday.

Paolo

I have two questions:

1) why isn't the existing check in kvm_irqchip_get_virq enough to fix the bug?

    if (!s->direct_msi && retry) {
        retry = false;
        kvm_flush_dynamic_msi_routes(s);
        goto again;
    }

2) If you introduce this extra call to kvm_flush_dynamic_msi_routes, does the existing check become obsolete?

Thanks,

Paolo