[Qemu-devel] [PATCH] [KVM] Needless to update msi route when only msi-x entry "control" section changed

[Zhanghaoyu (A)]

With regard to old version linux guest(e.g., rhel-5.5), in ISR processing, mask 
and unmask msi-x vector every time, which result in VMEXIT, then QEMU will 
invoke kvm_irqchip_update_msi_route() to ask KVM hypervisor to update the VM 
irq routing table. In KVM hypervisor, synchronizing RCU needed after updating 
routing table, so much time consumed for waiting in wait_rcu_gp(). So CPU usage 
in VM is so high, while from the view of host, VM's total CPU usage is so low. 
Masking/unmasking msi-x vector only set msi-x entry "control" section, needless 
to update VM irq routing table.

Signed-off-by: Zhang Haoyu <address@hidden>
Signed-off-by: Huang Weidong <address@hidden>
Signed-off-by: Qin Chuanyu <address@hidden>
---
hw/i386/kvm/pci-assign.c | 3 +++
1 files changed, 3 insertions(+)

--- a/hw/i386/kvm/pci-assign.c  2013-05-04 15:53:18.000000000 +0800
+++ b/hw/i386/kvm/pci-assign.c  2013-05-04 15:50:46.000000000 +0800
@@ -1576,6 +1576,8 @@ static void assigned_dev_msix_mmio_write
                 MSIMessage msg;
                 int ret;

+                /* Needless to update msi route when only msi-x entry 
"control" section changed */
+                if ((addr & (PCI_MSIX_ENTRY_SIZE - 1)) != 
PCI_MSIX_ENTRY_VECTOR_CTRL){
                 msg.address = entry->addr_lo |
                     ((uint64_t)entry->addr_hi << 32);
                 msg.data = entry->data;
@@ -1585,6 +1587,7 @@ static void assigned_dev_msix_mmio_write
                 if (ret) {
                     error_report("Error updating irq routing entry (%d)", ret);
                 }
+                }
             }
         }
     }

Thanks,
Zhang Haoyu