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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.05.18 at 15:35, <aisaila@xxxxxxxxxxxxxxx> wrote:
> 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?

I may not correctly understand the question: I think everybody agrees
on the bits to go into an _unused_ portion of the p2m entry. 

Jan



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