Re: [Xen-devel] [PATCH 2/2] xen/mm: Introduce PG_state_uninitialised

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 18 March 2020 10:10
> To: paul@xxxxxxx
> Cc: 'David Woodhouse' <dwmw2@xxxxxxxxxxxxx>; sstabellini@xxxxxxxxxx; 
> julien@xxxxxxx; wl@xxxxxxx;
> konrad.wilk@xxxxxxxxxx; george.dunlap@xxxxxxxxxxxxx; 
> andrew.cooper3@xxxxxxxxxx;
> ian.jackson@xxxxxxxxxxxxx; george.dunlap@xxxxxxxxxx; 
> jeff.kubascik@xxxxxxxxxxxxxxx; 'Xia, Hongyan'
> <hongyxia@xxxxxxxxxx>; stewart.hildebrand@xxxxxxxxxxxxxxx; 
> xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH 2/2] xen/mm: Introduce PG_state_uninitialised
> 
> On 18.03.2020 09:53, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of 
> >> David Woodhouse
> >> Sent: 17 March 2020 22:15
> >>
> >> On Thu, 2020-02-20 at 12:59 +0100, Jan Beulich wrote:
> >>> On 07.02.2020 19:04, David Woodhouse wrote:
> >>>> --- a/xen/arch/x86/mm.c
> >>>> +++ b/xen/arch/x86/mm.c
> >>>> @@ -488,7 +488,8 @@ void share_xen_page_with_guest(struct page_info 
> >>>> *page, struct domain *d,
> >>>>
> >>>>       page_set_owner(page, d);
> >>>>       smp_wmb(); /* install valid domain ptr before updating refcnt. */
> >>>> -    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
> >>>> +    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
> >>>> +           (page->count_info & ~PGC_xen_heap) == 
> >>>> PGC_state_uninitialised);
> >>>
> >>> Can uninitialized pages really make it here?
> >>
> >> Yep, we share the low 1MiB with dom_io.
> >>
> >
> > OOI anyone know why we do this? Is it actually necessary?
> 
> Yes, for Dom0 to be able to access things like EBDA, IBFT, or data
> found in BIOS space.
>

Ok. I am still wondering why dom0's low 1MiB of pfn space is not simply mapped 
1:1 though. Just historical?

  Paul
 
> Jan


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