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

[Radim Krčmář]

2016-08-09 16:19+0800, Chao Gao:
> 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.
> 
> ok, after adding "x2apic_phys" to guest kernel boot parameter, I 
> boot up a 288(yes, 288) vcpus guest successfully with command
> qemu-system-x86_64 -boot c -m 4096 -sdl --enable-kvm \
> -M kernel-irqchip=split -bios bios.bin -smp cpus=288 -hda vdisk.img \
> -device intel-iommu,intremap=on -machine q35
> Also, I can see interrupts on those cpu with inital apicid>255 from 
> /proc/cpuinfo and /proc/interrupts. 

Great, thanks for testing!
Only IPIs will be correctly delivered to apic_id > 255 without few more
patches on the QEMU side, though.