Re: [V6 PATCH 6/7] pvh dom0: Add and remove foreign pages

[Mukesh Rathor <mukesh.rathor@oracle.com>]

On Fri, 06 Dec 2013 11:46:35 +0000
"Jan Beulich" <JBeulich@suse.com> wrote:

> >>> On 06.12.13 at 03:54, Mukesh Rathor <mukesh.rathor@oracle.com>
> >>> wrote:
> > @@ -693,11 +695,42 @@ long do_memory_op(unsigned long cmd,
> > XEN_GUEST_HANDLE_PARAM(void) arg) return rc;
> >          }
> >  
> > +        /*
> > +         * If autotranslate guest, (eg pvh), the gfn could be
> > mapped to a mfn
> > +         * from foreign domain by the user space tool during
> > domain creation.
> > +         * We need to check for that, free it up from the p2m, and
> > release
> > +         * refcnt on it. In such a case, page would be NULL and
> > the following
> > +         * call would not have refcnt'd the page.
> > +         * See also xenmem_add_foreign_to_p2m().
> > +         */
> >          page = get_page_from_gfn(d, xrfp.gpfn, NULL, P2M_ALLOC);
> >          if ( page )
> > +            mfn = page_to_mfn(page);
> > +#ifdef CONFIG_X86
> 
> I take this to mean that the code is okay for ARM now. But such a
> conditional here needs explanation in a code comment, or putting
> into something that's generic (i.e. "else if ()") but currently
> happening to be always false for ARM.
> 
> > +        else
> >          {
> > -            guest_physmap_remove_page(d, xrfp.gpfn,
> > page_to_mfn(page), 0);
> > -            put_page(page);
> > +            mfn = mfn_x(get_gfn_query(d, xrfp.gpfn, &p2mt));
> > +            if ( p2m_is_foreign(p2mt) )
> > +            {
> > +                struct domain *foreign_dom;
> > +
> > +                foreign_dom = page_get_owner(mfn_to_page(mfn));
> > +                ASSERT(is_pvh_domain(d));
> > +                ASSERT(d != foreign_dom);
> > +            }
> > +        }
> > +#endif
> > +        if ( page || p2m_is_foreign(p2mt) )
> > +        {
> > +            guest_physmap_remove_page(d, xrfp.gpfn, mfn, 0);
> > +            if ( page )
> > +                put_page(page);
> > +
> > +            if ( p2m_is_foreign(p2mt) )
> > +            {
> > +                put_page(mfn_to_page(mfn));
> > +                put_gfn(d, xrfp.gpfn);
> > +            }
> 
> The code as it stands gives the impression that there could be two
> put_page() invocations in a single run here. Based on the comment
> above I assume this should never be the case though. That would
> be nice to be documented via a suitable ASSERT(), or it could be
> made more obvious by doing something like
> 
>         if ( page || p2m_is_foreign(p2mt) )
>         {
>             guest_physmap_remove_page(d, xrfp.gpfn, mfn, 0);
>             if ( p2m_is_foreign(p2mt) )
>             {
>                 page = mfn_to_page(mfn);
>                 put_gfn(d, xrfp.gpfn);
>             }
>             put_page(page);
>         }

Good idea to just have one put_page. But I reworked the code to make
xenmem_rem_foreign_from_p2m like Ian C suggested. I think that may be
the cleanest solution. Please take a look.

thanks
Mukesh