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

[Julien Grall <julien.grall@xxxxxxx>]

On 08/08/2019 14:03, Oleksandr wrote:
> Hi, Julien, Roger.
Hi,

> > > > 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?
I can't see how iommu_hardware_setup() can be called on staging when iommu=0 as 
this is protected by a if ( iommu_enable ).
Can you please give a stack trace how this is called and the version you use? 
WARN() should do it for you.
Cheers,

--
Julien Grall

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