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

[Oleksandr <olekstysh@xxxxxxxxx>]

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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