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

[Eric Auger]

Hi Evgeny,
On 1/16/23 16:37, Evgeny Iakovlev wrote:
> 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?
Personally I don't have anything against. I guess we cannot prevent you
from adding a feature that is supported by the spec ;-) One concern I
have is the extra complexity it will bring to the device. At least I
would recommend you to try to isolate stage2 support at most from the
rest and make your utmost to keep the stage1 perf as close as possible
to what it is currently. Indeed the main "production" use case is S1
vSMMU with KVM acceleration. Currently in linux arm-smmuv3 driver, only
S1 is used. Out of curiosity how are the S2 and S1+S2 used with HyperV?

I am mostly involved in KVM accelerated use cases so I will let others
comment on potential use cases using S2.
>
> * Is there anyone already working on it as we speak maybe?
afaik I am not aware of anybody working on that at the moment
>
> * Were there any previous attempts to do this and if yes why they
> evidently didn't get through?
afaik there were no series sent publicly

Thanks

Eric
>
>
> Thanks!
>
>