Creating a new, pass-through, host and guest

[xvisor]

Creating a new, pass-through, host and guest

[avpatel]

Hi Tom,

For pass-through of any device we need two things:

1. Host-to-Guest IRQ routing for the device
2. SysMMU support in HW for the device if device is DMA capable (or Bus Master)

Year 2011
You are right we had pass-through support in Xvisor with few example DTS files. This pass-through support was only for non-DMA capable devices because we did not have SMMU support. We had tested this pass-through support with PL011 (uart) and LAN9118 (ethernet) pass-through. Both these devices are non-DMA capable devices. The key part of this pass-through support was Host-to-Guest IRQ routing in SW emulated GIC (i.e. emulators/pic/gic.c)

Year 2012/2013
We realized that supporting only non-DMA capable devices was of not much worth. Also, we discovered that Host-to-Guest IRQ routing did not work well on ARM fast models. Due to this reasons we depricated Host-to-Guest IRQ routing support in SW emulated GIC (i.e. emulators/pic/gic.c).

Year 2014
We are back with pass-through support in Xvisor along with example DTS files. This time we have used GIC virtualization extension for Host-to-Guest IRQ routing. Our HW assisted GIC emulator (i.e. arch/arm/common/vgic.c) support Host-to-Guest IRQ routing. This works quite well on ARM Fast Model and Foundation v8 Model. We also framework for meta emulators for pass-through devices (which other hypervisors don't have). We still lack SMMU support but with meta emulator for pass-through device it should quite easy to add it now.
(Example DTS files for UART pass-through <xvisor_next_source>/arch/arm/board/generic/dts/vexpress/a15/two_pt_guest_vexpress-a15.dts)

Now for OMAP3:

1. We don't have VGIC or GIC HW virtualization extension hence latest Xvisor wont be able to support Host-to-Guest IRQ routing. We will need to come-up new method for Host-to-Guest IRQ routing in SW emulated GIC (i.e. emulators/pic/gic.c) which works fine on ARM Fast Model and real HW.
2. We don't have SMMU in OMAP3 so we won't able to pass-through DMA-capable devices even if we have Host-to-Guest IRQ routing in emulators/pic/gic.c. As a work-around for this we need to change the Guest OS to see Guest Physical Address == Host Physical Address for RAM regions but this won't be a clean approach.

My recommendations:
If pass-through device is extremely important for you then cleanest way would be to use SOC having HW virtualization support and SMMU. If you want to stick with OMAP3 then I can help you address point1 and point2 above for OMAP3 but this will become specific to your application.

Regards,
Anup

[TomHennen]

This makes sense. I'll consider my options.

Thanks so much for the thoughtful response.