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 7:05 AM Tamas K Lengyel
<tamas.k.lengyel@xxxxxxxxx> 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.

A potential solution for this - if one would need it in the future -
would be to add another VM event type that Xen can use to alert the
toolstack that a pre-existing mem_access permission is being
overwritten by forking. That would allow the toolstack to reset the
permission if it wants to. But honestly, I think it's a lot of code
for a situation that I don't expect anyone will ever run into. Let's
just document it and move on.

Tamas

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