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 04:02 PM, Stefano Stabellini wrote:
> > 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.
> 
> The cases are not so distinct. We need the 1:1 grant mapping for the
> IOMMU because of the swiotlb and the 1:1 memory mapping.
> 
> Without it we can easily bypass this code.

Only until we have just one smmu driver with shared pagetables.
Otherwise the two cases are different.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel