Re: [Xen-devel] [PATCH v1 1/2] x86/mm: Add mem access rights to NPT

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On Ma, 2018-05-22 at 03:49 -0600, Jan Beulich wrote:
> >
> > >
> > > >
> > > > On 18.05.18 at 20:30, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> > On 05/18/2018 06:30 PM, Jan Beulich wrote:
> > >
> > > >
> > > > >
> > > > > >
> > > > > > On 11.05.18 at 13:11, <aisaila@xxxxxxxxxxxxxxx> wrote:
> > > > This patch adds access rights for the NPT pages. The access
> > > > rights are
> > > > saved in bits 59:56 of pte that are manipulated through
> > > > p2m_set_access()
> > > > and p2m_get_access() functions.
> > > You don't give any rationale for the choice of bits. Right now
> > > p2m-pt.c still
> > > assumes that CPU and IOMMU page tables might be shared, despite
> > > amd_iommu_init() unconditionally turning this functionality off.
> > > As long as the
> > > option for that mode hasn't been removed from p2m-pt.c, I think
> > > bits used
> > > by the IOMMU (here: bit 59) should not be used for software
> > > purposes. The
> > > alternative therefore is for you to supply a prereq patch purging
> > > the sharing
> > > functionality from p2m-pt.c and preferably also from the AMD
> > > IOMMU code.
> > > That's of course only an option if we don't foresee any means by
> > > which this
> > > mode may become usable again.
> > The choice of bits was our interpretation of Andrew's reply here:
> >
> > https://lists.xenproject.org/archives/html/xen-devel/2018-05/msg005
> > 73.html
> >
> > Have we misread it?
> I don't think you have, but what Andrew has described was only the
> CPU side
> of considerations to make. Plus of course the patch description
> should explain
> whatever choice you make.
>
> >
> > We've also thought about putting the information in a new field of
> > struct page_info. Would that perhaps be preferable?
> I don't view this as a page property, but a mapping property. As such
> it can't validly go into struct page_info.

I will add the information in the patch description. Can you tell us
what structure is best to use for the access rights?

Thanks,
Alex

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