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 Tue, 2012-11-27 at 00:47 +0800, yi li wrote:
> hi Alex,
> 
> the qemu-kvm version 1.2.

And is the device making use of MSI-X or MSI interrupts.  MSI-X should
work on 1.2, MSI does not yet support vector updates for affinity, but
patches are welcome.  Thanks,

Alex

> 2012/11/26 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
> >