Re: Enable more than 255 VCPU support without irq remapping function in the guest

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2016-04-26 18:14, Lan, Tianyu wrote:
> Hi All:
> 
> Recently I am working on extending max vcpu to more than 256 on the both
> KVM/Xen. For some HPC cases, it needs many vcpus. The job requires to
> use X2APIC in the guest which supports 32-bit APIC id. Linux kernel
> requires irq remapping function during enabling X2APIC when max APIC id
> is more than 255(More detail please see try_to_enable_x2apic()).
> 
> The irq remapping function helps to deliver irq to cpu 255~. IOAPIC just
> supports 8-bit target APIC id field and only can deliver irq to
> cpu 0~255.
> 
> So far both KVM/Xen doesn't enable irq remapping function. If enable the
> function, it seems a huge job which need to rework IO-APIC, local APIC,
> MSI parts and add virtual VTD support in the KVM.
> 
> Other quick way to enable more than 256 VCPUs is to eliminate the
> dependency between irq remapping and X2APIC in the guest linux kernel.
> So far I can boot the guest after removing the dependency.
> The side effect I thought is that irq only can deliver to 0~255 vcpus
> but 256 vcpus seem enough to balance irq requests in the guest. In the
> most cases, there are fewer devices in the guest.
> 
> I wonder whether it's feasible. There maybe some other side effects I
> didn't think of. Very appreciate for your comments.

Radim is working on the KVM side already, Peter is currently driving the
VT-d interrupt emulation topic in QEMU. It's in reach, I would say. :)

Jan

PS: Please no PV mess, at least without good reasons.

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