[RFC] 2-stage translation emulation support for SMMUv3 on TCG

[Evgeny Iakovlev]

Hi!


We are using qemu-tcg-aarch64 to run Hyper-V test and debug builds for 
arm. Besides some minor fixes that i have submitted over the last couple 
of weeks, one big compatibility item for us is SMMUv3 2-stage 
translations support. We can do fine without it right now, but having it 
would also allow us to test nested arm guests with SMMUv3, which is great.


One idea we have floating around is implementing 2-stage translations in 
SMMUv3 in Qemu. We can't make a commitment yet, but before we consider 
it i think it would be wise to ask the community about it, specifically:

* Do 2-stage translations sound like something qemu-arm would be keen on 
accepting? Are there any other use-cases for it besides an arguably wild 
corner case of nesting an EL2 hypervisor on software-emulated arm64?

* Is there anyone already working on it as we speak maybe?

* Were there any previous attempts to do this and if yes why they 
evidently didn't get through?


Thanks!