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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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.
In fact the two are not the same.

On ARM:

#define gnttab_need_iommu_mapping(d)                    \
    (is_domain_direct_mapped(d) && need_iommu(d))

#ifdef HAS_PASSTHROUGH
#define need_iommu(d)    ((d)->need_iommu)
#else
#define need_iommu(d)    (0)
#endif

Also need_iommu is only set if an iommu is available.

On the other hand:

#define gnttab_need_identity_mapping(d)                    \
    (is_domain_direct_mapped(d) && !need_iommu(d))

So gnttab_need_identity_mapping would cover the case of a direct_mapped
dom0 with an iommu available.

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