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

[Alexandru Stefan ISAILA <aisaila@xxxxxxxxxxxxxxx>]

On Mi, 2018-05-30 at 14:56 +0100, Andrew Cooper wrote:
> On 30/05/18 12:29, Jan Beulich wrote:
> >
> > >
> > > >
> > > > >
> > > > > On 30.05.18 at 13:20, <aisaila@xxxxxxxxxxxxxxx> wrote:
> > > On Mi, 2018-05-30 at 03:52 -0600, Jan Beulich wrote:
> > > >
> > > > >
> > > > > >
> > > > > > >
> > > > > > > On 30.05.18 at 11:04, <aisaila@xxxxxxxxxxxxxxx> wrote:
> > > > > Sorry for the misunderstanding, I wanted to clarify if the
> > > > > 59:56
> > > > > bits
> > > > > are fully ok to be used or if not then where should I use 4
> > > > > bits to
> > > > > store the mem access info?
> > > > I thought I had sufficiently explained this - you have two
> > > > options:
> > > > 1) Make sure (via some prereq patch(es)) bit 59 has no other
> > > > use, and
> > > >    then use 59:56.
> > > > 2) Use another range that's provably having no other use, e.g.
> > > >    58:55.
> > > I've checked and bits 40:52 are defined in asm/page.h for page
> > > flags.
> > 40:52? Hardly.
> >
> > >
> > > I've tried bits 53:56 and there where some problems with the
> > > guest not
> > > starting or the image freezing,
> > Well, you'll have to explain this (perhaps just to yourself).
> >
> > >
> > > bits 62 and 63 are not free so 59:56 is
> > > the only space to be used for this purpose and is seems to
> > > function
> > > correctly.
> > Well - as said before, bit 59 is not available for use without some
> > prereq work.
> There are no software available bits in the top of an AMD IOMMU PTE.
> Bits 59:62 are defined, while bits 52:58 are strictly reserved and
> fault
> if used.
>
> I'm also not convinced of the safety of our current uses of bits 9:11
> which are software available in the regular pagetables, but have
> specific meaning in the IOMMU entries.
>
> If the code IOMMU code disables page sharing, then lets go one small
> step further and prohibit its use entirely.  There is no point trying
> to
> maintain compatibility for an option which isn't used, especially if
> it
> gets in the way of improvements like this in the SVM code.
>
Another idea is to save the mem access info in a radix tree like on the
ARM side and we can store the radix tree root in the p2m_domain.

I think that this is the fastest and cleanest way to solve the free
bits problem.

Is this a suitable way to go?


Thanks,
Alex

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