Re: [Xen-devel] Discussion about virtual iommu support for Xen guest

[Yang Zhang <yang.zhang.wz@xxxxxxxxx>]

On 2016/5/26 16:29, Lan Tianyu wrote:
> Hi All:
> We try pushing virtual iommu support for Xen guest and there are some
> features blocked by it.
>
> Motivation:
> -----------------------
> 1) Add SVM(Shared Virtual Memory) support for Xen guest
> To support iGFX pass-through for SVM enabled devices, it requires
> virtual iommu support to emulate related registers and intercept/handle
> guest SVM configure in the VMM.
IIRC, SVM needs the nested IOMMU support not only virtual iommu. Correct 
me if i am wrong.
> 2) Increase max vcpu support for one VM.
>
> So far, max vcpu for Xen hvm guest is 128. For HPC(High Performance
> Computing) cloud computing, it requires more vcpus support in a single
> VM. The usage model is to create just one VM on a machine with the
> same number vcpus as logical cpus on the host and pin vcpu on each
> logical cpu in order to get good compute performance.
>
> Intel Xeon phi KNL(Knights Landing) is dedicated to HPC market and
> supports 288 logical cpus. So we hope VM can support 288 vcpu
> to meet HPC requirement.
>
> Current Linux kernel requires IR(interrupt remapping) when MAX APIC
> ID is > 255 because interrupt only can be delivered among 0~255 cpus
> without IR. IR in VM relies on the virtual iommu support.
>
> KVM Virtual iommu support status
> ------------------------
> Current, Qemu has a basic virtual iommu to do address translation for
> virtual device and it only works for the Q35 machine type. KVM reuses it
> and Redhat is adding IR to support more than 255 vcpus.
>
> How to add virtual iommu for Xen?
> -------------------------
> First idea came to my mind is to reuse Qemu virtual iommu but Xen didn't
> support Q35 so far. Enabling Q35 for Xen seems not a short term task.
> Anthony did some related jobs before.
>
> I'd like to see your comments about how to implement virtual iommu for Xen.
>
> 1) Reuse Qemu virtual iommu or write a separate one for Xen?
> 2) Enable Q35 for Xen to reuse Qemu virtual iommu?
>
> Your comments are very appreciated. Thanks a lot.

--
best regards
yang

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