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

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

On 6/8/2016 4:11 PM, Tian, Kevin wrote:
> It makes sense... I thought you used this security issue against
> placing vIOMMU in Qemu, which made me a bit confused earlier. :-)
>
> We are still thinking feasibility of some staging plan, e.g. first
> implementing some vIOMMU features w/o dependency on root-complex in
> Xen (HVM only) and then later enabling full vIOMMU feature w/
> root-complex in Xen (covering HVMLite). If we can reuse most code
> between two stages while shorten time-to-market by half (e.g. from
> 2yr to 1yr), it's still worthy of pursuing. will report back soon
> once the idea is consolidated...
>
> Thanks Kevin

After discussion with Kevin, we draft a staging plan of implementing
vIOMMU in Xen based on Qemu host bridge. Both virtual devices and
passthough devices use one vIOMMU in Xen. Your comments are very 
appreciated.
1. Enable Q35 support in the hvmloader.
In the real world, VTD support starts from Q35 and OS may have such
assumption that VTD only exists on the Q35 or newer platform.
Q35 support seems necessary for vIOMMU support.

In regardless of Q35 host bridge in the Qemu or Xen hypervisor,
hvmloader needs to be compatible with Q35 and build Q35 ACPI tables.

Qemu already has Q35 emulation and so the hvmloader job can start with
Qemu. When host bridge in Xen is ready, these changes also can be reused.

2. Implement vIOMMU in Xen based on Qemu host bridge.
Add a new device type "Xen iommu" in the Qemu as a wrapper of vIOMMU
hypercalls to communicate with Xen vIOMMU.

It's in charge of:
1) Query vIOMMU capability(E,G interrupt remapping, DMA translation, SVM
and so on)
2) Create vIOMMU with predefined base address of IOMMU unit regs
3) Notify hvmloader to populate related content in the ACPI DMAR
table.(Add vIOMMU info to struct hvm_info_table)
4) Deal with DMA translation request of virtual devices and return
back translated address.
5) Attach/detach hotplug device from vIOMMU


New hypercalls for vIOMMU that are also necessary when host bridge in Xen.
1) Query vIOMMU capability
2) Create vIOMMU(IOMMU unit reg base as params)
3) Virtual device's DMA translation
4) Attach/detach hotplug device from VIOMMU


All IOMMU emulations will be done in Xen
1) DMA translation
2) Interrupt remapping
3) Shared Virtual Memory (SVM)

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