Re: [Xen-devel] Xen virtual IOMMU high level design doc V2

["Lan, Tianyu" <tianyu.lan@xxxxxxxxx>]

On 10/26/2016 5:36 PM, Jan Beulich wrote:
> > > > On 18.10.16 at 16:14, <tianyu.lan@xxxxxxxxx> wrote:
> > 1.1 Enable more than 255 vcpu support
> > HPC cloud service requires VM provides high performance parallel
> > computing and we hope to create a huge VM with >255 vcpu on one machine
> > to meet such requirement.Ping each vcpus on separated pcpus. More than
> > 255 vcpus support requires X2APIC and Linux disables X2APIC mode if
> > there is no interrupt remapping function which is present by vIOMMU.
> > Interrupt remapping function helps to deliver interrupt to #vcpu >255.
> > So we need to add vIOMMU before enabling >255 vcpus.
> I continue to dislike this completely neglecting that we can't even
> have >128 vCPU-s at present. Once again - there's other work to
> be done prior to lack of vIOMMU becoming the limiting factor.
Yes, we can increase vcpu from 128 to 255 first without vIOMMU support.
We have some draft patches to enable this. Andrew also will rework CPUID
policy and change the rule of allocating vcpu's APIC ID. So we will base
on it to increase vcpu number. VLAPIC also needs to be changed to
support >255 APIC ID. These jobs can be implemented parallel with vIOMMU.

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