[Qemu-devel] [PATCH] Fix irq route entries exceed KVM_MAX_IRQ_ROUTES

["kevinnma(马文霜)" <kevinnma@tencent.com>]

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.

Signed-off-by: Wenshuang Ma <kevinnma@tencent.com>
---
 kvm-all.c |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/kvm-all.c b/kvm-all.c
index 18cc6b4..f47e1b1 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -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