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 7:44 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 29.01.2020 15:05, Tamas K Lengyel wrote:
> > 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.
>
> I think I would ack a revised patch having this properly explained.

That's fine, I'll add some comments to this effect and reword the
commit message.

Tamas

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