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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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/msg00573.html

Have we misread it?

We've also thought about putting the information in a new field of
struct page_info. Would that perhaps be preferable?


Thanks,
Razvan

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