Re: [Xen-devel] [PATCH v2 1/2] xen: introduce arch_iommu_grant_(un)map_page

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 24 Jul 2014, Julien Grall wrote:
> On 24/07/14 12:07, Stefano Stabellini wrote:
> > On Thu, 24 Jul 2014, Julien Grall wrote:
> > > Hi Stefano,
> > > 
> > > The Title of the commit message is now wrong.
> > 
> > Thanks, I didn't notice.
> > 
> > 
> > > On 23/07/14 18:19, Stefano Stabellini wrote:
> > > > Introduce two arch specific functions to create a new p2m mapping of
> > > > granted pages at pfn == mfn.
> > > > The x86 implementation just returns ENOSYS.
> > > > 
> > > > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > > > ---
> > > >    xen/arch/arm/p2m.c        |   19 +++++++++++++++++++
> > > >    xen/include/asm-arm/p2m.h |    4 ++++
> > > >    xen/include/asm-x86/p2m.h |   13 +++++++++++++
> > > >    3 files changed, 36 insertions(+)
> > > > 
> > > > diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> > > > index 9960e17..c38af59 100644
> > > > --- a/xen/arch/arm/p2m.c
> > > > +++ b/xen/arch/arm/p2m.c
> > > > @@ -555,6 +555,25 @@ void guest_physmap_remove_page(struct domain *d,
> > > >                          pfn_to_paddr(mfn), MATTR_MEM, p2m_invalid);
> > > >    }
> > > > 
> > > > +int arch_grant_map_page_identity(struct domain *d, unsigned long frame,
> > > > +                                 bool_t writeable)
> > > > +{
> > > > +    p2m_type_t t;
> > > > +
> > > > +    if ( writeable )
> > > > +        t = p2m_ram_rw;
> > > > +    else
> > > > +        t = p2m_ram_ro;
> > > 
> > > This is not the right p2m type to use here. p2m_ram_{rw,ro} allow foreign
> > > mapping. So another guest could access to the grant.
> > > 
> > > I would use p2m_iommu_map_{rw,ro}.
> > 
> > 
> > I see. I'll make the change and add a comment to explain why we are
> > using p2m_iommu types for non-iommu related mappings.
> 
> The code to add the 1:1 mapping for the SMMU is very ugly. I was wondering if
> we could drop the iommu_map_page callback and
> directly use arch_grant_map_page_identity in all the case.
> 
> It would avoid the if/else in the grant code.

Thing is arch_grant_map_page_identity has nothing to do with IOMMUs. It
gets confusing, especially on x86 where people don't know why we need
this. The if/else is ugly but clearer.
See also the previous email exchange with Jan.

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