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

[Oleksandr <olekstysh@xxxxxxxxx>]

On 08.08.19 14:01, Roger Pau Monné wrote:

Hi, 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.
> >
> > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> > ---
> > I have heard there is a "possible" case when the IOMMU could be accessible by 
> > DOM0.
> > So, I think, for this to work we need to create corresponding DT nodes in the DT
> > at least.
> dom0 on ARM being an autotranslated guest I'm not sure how it's going
> to program the DMA remapping in the iommu, since it doesn't know the
> mfns of the memory it uses at all, hence I don't see the point in
> exposing the hardware iommu to dom0 unless there's some emulation done
> to make dom0 able to access it.
Currently, Dom0 on ARM is always 1:1 mapped (gfn == mfn). However, I 
don't really know how long this assumption it is going to be true.
Being honest, I don't need this use-case at the moment. I have 
experimented with the code which creates DT nodes for Dom0 a bit, and 
recollected
about the possibility for the IOMMU to be accessible by Dom0. If this 
patch doesn't have any real value, I would be ok to just drop it.
> Thanks, Roger.
--
Regards,

Oleksandr Tyshchenko


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