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

[Oleksandr <olekstysh@xxxxxxxxx>]

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.

iommu_hardware_setup() is not called. But, devices have DEVICE_IOMMU 
set, even if "iommu=0". I am based on "7d1460c xen/arm: optee: fix 
compilation with GCC 4.8" + Stefano's reserved-memory series + my IPMMU 
series.

--
Regards,

Oleksandr Tyshchenko


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