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Re: [Xen-devel] [PATCH] xen/arm: Let the IOMMU be accessible by Dom0 if forcibly disabled in Xen




Hi, Julien, Roger.





On Thu, Aug 08, 2019 at 01:53:23PM +0300, Oleksandr Tyshchenko wrote:
From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

Don't skip IOMMU nodes when creating DT for Dom0 if IOMMU has been
forcibly disabled in bootargs (e.g. "iommu=0") in order to let
the IOMMU be accessible by DOM0.

I don't think your code is doing what you expect... If iommu=0, then Xen will not lookup for IOMMUs (iommu_hardware_setup() will not be called). So none of the device will have DEVICE_IOMMU set and hence they are already given to dom0.

But I think it is wrong to give the IOMMUs to Dom0 when iommu=0. This is not the goal of this option. If you want to passthrough the IOMMU to Dom0, then you should use the parameter iommu_hwdom_passthrough.

However, I agree with Roger that giving the IOMMU to dom0 is a pretty bad idea. So this should be fixed.


I fully agree with the arguments provided that it is a bad idea. So, please consider that patch as not relevant.


But, I am not sure I follow the last sentence:

>>> If iommu=0, then Xen will not lookup for IOMMUs (iommu_hardware_setup() will not be called). So none of the device will have DEVICE_IOMMU set and hence they are already given to dom0.

I can see that devices have DEVICE_IOMMU set. Although, the IOMMU driver is not in use, it is present and compatible matches. So, even if iommu=0, the IOMMU devices are not given to Dom0, because of skipped. Or I missed something?


--
Regards,

Oleksandr Tyshchenko


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