Re: [Xen-devel] [PATCH v6 1/9] x86/p2m: Allow p2m_get_page_from_gfn to return shared entries

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

On Tue, Jan 28, 2020 at 9:38 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 27.01.2020 19:06, Tamas K Lengyel wrote:
> > --- a/xen/arch/x86/mm/p2m.c
> > +++ b/xen/arch/x86/mm/p2m.c
> > @@ -574,11 +574,12 @@ struct page_info *p2m_get_page_from_gfn(
> >                  if ( fdom == NULL )
> >                      page = NULL;
> >              }
> > -            else if ( !get_page(page, p2m->domain) &&
> > -                      /* Page could be shared */
> > -                      (!dom_cow || !p2m_is_shared(*t) ||
> > -                       !get_page(page, dom_cow)) )
> > -                page = NULL;
> > +            else
> > +            {
> > +                struct domain *d = !p2m_is_shared(*t) ? p2m->domain : 
> > dom_cow;
> > +                if ( !get_page(page, d) )
> > +                    page = NULL;
> > +            }
> >          }
> >          p2m_read_unlock(p2m);
> >
> > @@ -594,8 +595,9 @@ struct page_info *p2m_get_page_from_gfn(
> >      mfn = get_gfn_type_access(p2m, gfn_x(gfn), t, a, q, NULL);
> >      if ( p2m_is_ram(*t) && mfn_valid(mfn) )
> >      {
> > +        struct domain *d = !p2m_is_shared(*t) ? p2m->domain : dom_cow;
> >          page = mfn_to_page(mfn);
> > -        if ( !get_page(page, p2m->domain) )
> > +        if ( !get_page(page, d) )
> >              page = NULL;
> >      }
> >      put_gfn(p2m->domain, gfn_x(gfn));
>
> Personally I would have preferred to go without new local variables
> here, but I'm not the maintainer of this code. However, at the very
> least blank lines would need inserting between declaration and
> statements. With at least this done (which could be done while
> committing)
>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> As an aside, I don't think the title accurately describes the
> change, since there's just one out of two cases which shared
> entries weren't taken care of. Similarly the description doesn't
> reflect this at all.

Well, for our use-case it is broken right now. So just because it's
not broken in another doesn't make the title/description incorrect.

Tamas

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