Re: [RFC PATCH] xen/gnttab: Store frame GFN in struct page_info on Arm

[Oleksandr <olekstysh@xxxxxxxxx>]

On 13.09.21 09:17, Jan Beulich wrote:

Hi Jan

> On 10.09.2021 09:52, Jan Beulich wrote:
> > On 10.09.2021 01:04, Oleksandr Tyshchenko wrote:
> > > @@ -731,11 +733,19 @@ static void p2m_put_l3_page(const lpae_t pte)
> > >        */
> > >       if ( p2m_is_foreign(pte.p2m.type) )
> > >       {
> > > -        mfn_t mfn = lpae_get_mfn(pte);
> > > -
> > >           ASSERT(mfn_valid(mfn));
> > >           put_page(mfn_to_page(mfn));
> > >       }
> > > +
> > > +#ifdef CONFIG_GRANT_TABLE
> > > +    /*
> > > +     * Check whether we deal with grant table page. As the grant table page
> > > +     * is xen_heap page and its entry has known p2m type, detect it and mark
> > > +     * the stored GFN as invalid.
> > > +     */
> > > +    if ( p2m_is_ram(pte.p2m.type) && is_xen_heap_mfn(mfn) )
> > > +        page_set_frame_gfn(mfn_to_page(mfn), INVALID_GFN);
> > > +#endif
> > I take it the write done is benign for other Xen heap pages? I suppose
> > this would want making very explicit, as such an assumption is easy to
> > go stale by entirely unrelated changes.
> >
> > I also wonder whether you really mean to include p2m_ram_ro pages here
> > as well.
> Actually I've meanwhile realized I should put my question here quite
> differently: Aren't you effectively introducing an M2P here for Arm,
> except that you artificially limit it to the Xen heap pages needed for
> the grant table?
Difficult to say, it might indeed look a bit close to M2P, so the answer 
to your question is more yes than no. But, I didn't have a plan to 
introduce M2P on Arm. It turned out that stashing GFN into page_info (as 
was suggested) avoided huge lookups, so we have got MFN->GFN in the end. 
The purpose of this patch was just to fix a potential issue with 
remapping grant-table frame on architecture without the M2P (Arm).

> Jan
--
Regards,

Oleksandr Tyshchenko