Re: [PATCH 5/6] x86/pv: map and unmap page tables in mark_pv_pt_pages_rdonly

[Hongyan Xia <hx242@xxxxxxx>]

On Tue, 2020-04-28 at 17:33 +0200, Jan Beulich wrote:
> On 17.04.2020 11:52, Hongyan Xia wrote:
> > --- a/xen/arch/x86/pv/dom0_build.c
> > +++ b/xen/arch/x86/pv/dom0_build.c
> > @@ -50,17 +50,17 @@ static __init void
> > mark_pv_pt_pages_rdonly(struct domain *d,
> >      unsigned long count;
> >      struct page_info *page;
> >      l4_pgentry_t *pl4e;
> > -    l3_pgentry_t *pl3e;
> > -    l2_pgentry_t *pl2e;
> > -    l1_pgentry_t *pl1e;
> > +    l3_pgentry_t *pl3e, *l3t;
> > +    l2_pgentry_t *pl2e, *l2t;
> > +    l1_pgentry_t *pl1e, *l1t;
> 
> I don't quite see why the new local variables get introduced:
> unmap_domain_page(), iirc, is quite fine with a non-page-
> aligned argument.

You are right, although in this function, where plXe points to may not
be the page we want to unmap. When plXe becomes aligned and points to a
new page, we actually want to unmap the page before it increments to an
aligned value.

Hongyan