Re: [Xen-devel] [PATCH v3 3/3] xen/arm: introduce XENFEAT_grant_map_identity

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 24 Jul 2014, Julien Grall wrote:
> On 07/24/2014 03:43 PM, Jan Beulich wrote:
> >>>> On 24.07.14 at 16:10, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> >> On Thu, 24 Jul 2014, Jan Beulich wrote:
> >>>>>> On 24.07.14 at 15:31, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> >>>> --- a/xen/common/grant_table.c
> >>>> +++ b/xen/common/grant_table.c
> >>>> @@ -727,7 +727,7 @@ __gnttab_map_grant_ref(
> >>>>  
> >>>>      double_gt_lock(lgt, rgt);
> >>>>  
> >>>> -    if ( gnttab_need_iommu_mapping(ld) )
> >>>> +    if ( gnttab_need_iommu_mapping(ld) || 
> >>>> gnttab_need_identity_mapping(ld) 
> >> )
> >>>
> >>> As before I think this change is pointless.
> >>
> >> I don't understand how you propose to solve the problem.
> > 
> > I think ARM's gnttab_need_iommu_mapping() needs to just be
> > is_domain_direct_mapped() then.

That's not correct either. It would work, but it is conceptually wrong.
From the ARM perspective we have 2 distinct cases to cover:
- we have an smmu and we need to create a mapping for it
- we don't have an smmu and we need an identity mapping


> So ARM people will think we need an iommu to add the identity mapping.
> Why don't we rename gnttab_need_iommu_mapping into
> gnttab_need_identity_mapping?
> 
> It would less confusing for both x86 and ARM.

I don't think that would be any better.
We have 2 distinct cases to cover, we need to treat them as such.

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