Re: [Xen-devel] [PATCH v6 5/9] x86/mem_sharing: use default_access in add_to_physmap

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Wed, Jan 29, 2020 at 6:27 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 28.01.2020 18:02, Tamas K Lengyel wrote:
> > On Tue, Jan 28, 2020 at 9:56 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >>
> >> On 27.01.2020 19:06, Tamas K Lengyel wrote:
> >>> When plugging a hole in the target physmap don't use the access permission
> >>> returned by __get_gfn_type_access as it can be non-sensical, leading to
> >>> spurious vm_events being sent out for access violations at unexpected
> >>> locations. Make use of p2m->default_access instead.
> >>
> >> As before, to me "can be non-sensical" is insufficient as a reason
> >> here. If it can also be a "good" value, it still remains unclear
> >> why in that case p2m->default_access is nevertheless the right
> >> value to use.
> >
> > I have already explained in the previous version of the patch why I
> > said "can be". Forgot to change the commit message from "can be" to
> > "is".
>
> Changing just the commit message would be easy while committing.
> But even with the change I would ask why this is. Looking at
> ept_get_entry() (and assuming p2m_pt_get_entry() will work
> similarly, minus the p2m_access_t which can't come out of the
> PTE just yet), I see
>
>     if ( is_epte_valid(ept_entry) )
>     {
>         *t = p2m_recalc_type(recalc || ept_entry->recalc,
>                              ept_entry->sa_p2mt, p2m, gfn);
>         *a = ept_entry->access;
>
> near its end. Which means even a hole can have its access field
> set. So it's still not clear to me from the description why
> p2m->default_access is uniformly the value to use. Wouldn't you
> rather want to override the original value only if it's
> p2m_access_n together with p2m_invalid or p2m_mmio_dm (but not
> paged-out pages)?

At this point I would just rather state that add_to_physmap only works
on actual holes, not with paged-out pages. In fact, I would like to
see mem_paging being dropped from the codebase entirely since it's
been abandoned for years and noone expressing any interest in keeping
it. In the interim I would rather not spend unnecessary cycles on
speculating about potential corner-cases of mem_paging when noone
actually uses it.

> Of course then the question is whether there
> wouldn't be an ambiguity with p2m_access_n having got set
> explicitly on the page. But maybe this is impossible for
> p2m_invalid / p2m_mmio_dm?

As far as mem_access permissions go, I don't know of any usecase that
would set mem_access permission on a hole even if by looks of it it is
technically possible. At this point I would rather just put this
corner-case's description in a comment.

Tamas

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