Re: The smp_affinity cannot work correctly on guest os when PCI passthrough device using msi/msi-x with KVM

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 2012-11-23 at 11:06 +0800, yi li wrote:
> Hi Guys,
> 
> there have a issue about smp_affinity cannot work correctly on guest
> os when PCI passthrough device using msi/msi-x with KVM.
> 
> My reason:
> pcpu will occur a lot of ipi interrupt to find the vcpu to handle the
> irq.  so the guest os will VM_EXIT frequelty. right?
> 
> if smp_affinity can work correctly on guest os,  the best way is that
> the vcpu handle the irq is cputune at the pcpu which handle the
> kvm:pci-bus irq on the host.but  unfortunly, i find that smp_affinity
> can not work correctly on guest os when msi/msi-x.
> 
> how to reproduce:
> 1: passthrough a netcard (Brodcom BCM5716S) to the guest os
> 
> 2: ifup the netcard, the card will use msi-x interrupt default, and close the
> irqbalance service
> 
> 3:  echo 4 > cat /proc/irq/NETCARDIRQ/smp_affinity, so we assume the vcpu2
> handle the irq.
> 
> 4: we have set <vcpupin vcpu='2' cpuset='1'/> and set the irq kvm:pci-bus to
> the pcpu1 on the host.
> 
> we think this configure will reduce the ipi interrupt when inject interrupt to
> the guest os. but this irq is not only handle on vcpu2. maybe it is
> not our expect。

What version of qemu-kvm/qemu are you using?  There's been some work
recently specifically to enable this.  Thanks,

Alex