Re: [Qemu-devel] [PATCH for-2.8 00/18] pc: q35: x2APIC support in kvm_apic mode

[Peter Xu]

On Tue, Aug 09, 2016 at 08:33:13AM +0200, Jan Kiszka wrote:
> On 2016-08-09 08:24, Peter Xu wrote:
> > On Tue, Aug 09, 2016 at 02:18:15PM +0800, Peter Xu wrote:
> >> On Tue, Aug 09, 2016 at 12:33:17PM +0800, Chao Gao wrote:
> >>> On Mon, Aug 08, 2016 at 04:57:14PM +0800, Peter Xu wrote:
> >>>> On Mon, Aug 08, 2016 at 03:41:23PM +0800, Chao Gao wrote:
> >>>>> HI, everyone.
> >>>>>
> >>>>> We have done some tests after merging this patch set into the lastest 
> >>>>> qemu
> >>>>> master. In kvm aspect, we use the lastest kvm linux-next branch. Here 
> >>>>> are
> >>>>> some problems we have met.
> >>>>>
> >>>>> 1. We can't boot up a 288 vcpus linux guest with CLI:
> >>>>> qemu-system-x86_64 -boot c -m 4096 -sdl -monitor pty --enable-kvm \
> >>>>> -M kernel-irqchip=split -serial stdio -bios bios.bin -smp cpus=288 \
> >>>>> -hda vdisk.img -device intel-iommu,intremap=on -machine q35.
> >>>>> The problem exists, even after we only assign 32 vcpus to the linux 
> >>>>> guest.
> >>>>> Maybe the output "do_IRQ: 146.113 No irq handler for vector (irq -1)" 
> >>>>> is a clue.
> >>>>> The output of qemu and kernel is in attachments. Do you have any idea
> >>>>> about the problem and how to solve it?
> >>>>
> >>>> IIUC, we need to wait for Radim's QEMU patches to finally enable 288
> >>>> vcpus?
> >>>>
> >>>> Btw, could you please try adding this to the QEMU cmdline when testing
> >>>> with 32 vcpus:
> >>>>
> >>>>  -global ioapic.version=0x20
> >>>>
> >>>> I see that you were running RHEL 7.2 guest with a default e1000. In
> >>>> that case, we may need to boost ioapic version to 0x20.
> >>>
> >>> It doesn't work. My host machine has 16 cpus. When I assign 4 or 8 vcpus 
> >>> to the guest
> >>> or 255 vcpus but set "kernel-irqchip=off", the guest work well. Maybe 
> >>> when irqchip
> >>> is in kernel, intremap can only handle situations that vcpus number is 
> >>> less than 
> >>> physical cpus'. Do you think it's right? 
> >>
> >> I don't think so. Vcpu number should not be related to host cpu
> >> numbers.
> >>
> >> I think the problem is with x2apic. Currently, x2apic is enabled in
> >> vIOMMU when kernel irqchip is used. This is problematic, since
> >> actually we are throughing away dest_id[31:8] without Radim's patches,
> >> meanwhile I see that by default x2apic is using cluster mode.
> >>
> >> In cluster mode, 8 bits will possibly not suffice (I think the reason
> >> why >17 vcpus will bring trouble is that each cluster has 16 vcpus,
> >> we'll have trouble if we have more than one cluster).
> >>
> >> To temporarily solve your issue, you should not only need "-global
> >> ioapic.version=0x20" in QEMU command line, but also add "x2apic_phys"
> >> to you guest kernel boot parameter, to force guest boot with x2apic
> >> physical mode (not cluster mode). Though this can only work for <255
> >> vcpus. IMHO we may still need to wait for Radim's patches to test >255
> >> case.
> > 
> > Not sure whether we should temporarily disable EIM by default for now
> > (provide an extra flag to optionally enable it)? Since it might break
> > guests with >17 vcpus.
> > 
> > CC Jan as well.
> 
> A switch for EIM would be fine for me if it helps.
> 
> To my understanding, the issue will be gone with an enhance KVM
> interface that we can then also detect via some cap (to flip the default
> again)?

Would you help explain how to do it?

Btw, if we have that switch, the default can go back to EIM mode along
with Radim's future patches.

Thanks,

-- peterx