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

[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. 

Thanks.
-Chao