Re: [Qemu-devel] KVM VM(rhel-5.5) %si is too high when TX/RX packets

[Zhanghaoyu (A)]

> I running a VM(RHEL-5.5) on KVM hypervisor(linux-3.8 + QEMU-1.4.1), and 
> direct-assign intel 82576 VF to the VM. When TX/RX packets on VM to the other 
> host via iperf tool, top tool result on VM shown that the %si is too high, 
> approximately 95% ~ 100%, but from the view of host, the VM's total CPU usage 
> is about 20% - 30%. And the throughput rate is approximately 200Mb/s, far 
> from the line rate 1Gb/s, And, I found  the hardirq rate is lower than normal 
> by running "watch -d -n 1 cat /proc/interrupts", I think it's caused by the 
> too high %si, because the NIC's hardirq was disabled during the softirq 
> process.
> Then, I direct-assign the intel 82576 to the VM, the same case happened too. 
> I found the intel 82576 and intel 82576 VF's interrupt mode are both 
> PCI-MSI-X.
> 
> And,
> I rmmod the igb driver, and, re-insmod the igb driver(igb-4.1.2) with the 
> parameter IntMode=0/1(0:legacy, 1:MSI, 2:MSI-x), the problem then gone, the 
> %si is approximately 20% -30%, and the throughput rate came to the line rate, 
> about 940Mb/s.
> I update the VM to RHEL-6.1, the problem disappeared too.
> And, I found a very strange thing, the VM's 82576VF's irq routing is set one 
> time on Vf's one interrupt received, so frequently.

With regard to rhel-5.5(linux-2.6.18), in ISR process, mask and unmask msi-x 
vector function msi_set_mask_bit() was invoked 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 
process needed after updating routing table, so much time consumed for waiting 
in wait_rcu_gp(). 
So %si in VM is so high, while from the view of host, VM's total CPU usage is 
so low. 

Why in ISR process, masking and unmasking msi-x vector is needed every time? 

Thanks,
Zhang Haoyu