Re: [Xen-devel] [RFC PATCH 8/9] iommu: Split iommu_hwdom_init() into arch specific parts

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

Sorry for the late answer.

On 23/03/17 12:40, Oleksandr Tyshchenko wrote:
> On Thu, Mar 23, 2017 at 11:08 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> > > > > On 22.03.17 at 19:40, <olekstysh@xxxxxxxxx> wrote:
> > > On Wed, Mar 22, 2017 at 5:54 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> > > > > > > On 15.03.17 at 21:05, <olekstysh@xxxxxxxxx> wrote:
> > > > The code being moved from here has survived the previous separation
> > > > of arch-independent from arch-specific code, and looking at the code
> > > > I also can't immediately see what's x86-specific here, so please make
> > > > the description explain why the code as is can't be used.
> > > You are right, there is nothing x86-specific here at first sight.
> > > The reason why I moved this code to x86 folder is that we don't need
> > > to retrieve IOMMU mappings on ARM this way. With need_iommu being
> > > explicitly set at the hardware domain creation time
> > > we just need to ask unshared IOMMU driver to allocate its page table
> > > to be ready to receive and process IOMMU mappings from P2M code.
> > >
> > > Other points that had prevented me from using it as is.
> > > If the hardware domain isn't 1:1 mapped we won't know gfn. IIRC, we
> > > can find mfn by gfn for particular domain on ARM, but not vise versa.
> > > Also this iommu_hwdom_init() is being called before allocating domain
> > > memory on ARM. What is the point?
> > > So, the d->page_list is empty during it execution.
> > In which case the question even more so is - why move the code
> > if it simply does nothing in your case?
I didn't move the code, because back then we were only planning to 
support shared page table (iommu_use_hap_pt(...) always returns true on 
ARM so far).
With more perspective, this code cannot work on ARM because of 
mfn_to_gmfn (we don't have an M2P). If we keep the code like that after 
this series, this will at least expose a bug (the helper always assume a 
direct mapping).
So I think moving the code is the right solution.

> Reasonable question. Before answering you I would like to clarify the reason
> why the iommu_hwdom_init() is being called before allocating domain
> memory on ARM.
> Is it a bug or there is an explanation for doing this.
No real reason, I didn't see a reason to call this function later on. I 
would be interested to know whether there is a latent bug.
Anyway, I think this is a good idea to fully initialize the IOMMU early 
for DOM0 as the builder will take care of assigning the non-PCI device 
protected.
Cheers,

--
Julien Grall

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