Re: [Xen-devel] x86: mapping from Dom0 to DomU

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

Hi, Jan!

On 04/21/2017 06:55 PM, Jan Beulich wrote:
> > > > On 21.04.17 at 15:04, <andr2000@xxxxxxxxx> wrote:
> > Hi, all!
> >
> > I am working on a zero-copy scenario for x86
> > and for that I am mapping pages from Dom0 to DomU
> > (yes, I know there are at least security concerns).
> >
> > Everything is just fine, e.g. I can map grefs from Dom0 in DomU
> > with gnttab_map_refs, until I try to mmap those pages in DomU
> > with vm_insert_page and Xen starts to complain:
> >
> > (XEN) mm.c:989:d1v0 pg_owner 1 l1e_owner 1, but real_pg_owner 0
> > (XEN) mm.c:1061:d1v0 Error getting mfn 20675a (pfn 1ac8de) from L1 entry
> > 800000020675a027 for l1e_owner=1, pg_owner=1
> >
> > So, the offending(?) code is at [1] which doesn't allow the mapping
> > I want. When removed with
> >
> > diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> > index f35e3116bb25..aeb93be8b529 100644
> > --- a/xen/arch/x86/mm.c
> > +++ b/xen/arch/x86/mm.c
> > @@ -980,6 +980,7 @@ get_page_from_l1e(
> >             * dom0, until pvfb supports granted mappings. At that time this
> >             * minor hack can go away.
> >             */
> > +#if 0
> >            if ( (real_pg_owner == NULL) || (pg_owner == l1e_owner) ||
> >                 xsm_priv_mapping(XSM_TARGET, pg_owner, real_pg_owner) )
> >            {
> > @@ -988,6 +989,7 @@ get_page_from_l1e(
> > real_pg_owner?real_pg_owner->domain_id:-1);
> >                goto could_not_pin;
> >            }
> > +#endif
> >            pg_owner = real_pg_owner;
> >        }
> > I can successfully mmap and use pages from Dom0 in DomU.
> >
> > Can anybody please explain why the use-case I am trying to implement
> > is treated as error from Xen's POV and what would be the right way to
> > do so?
> Granted pages can be mapped only through the grant-table hypercall,
If this is gnttab_map_refs call you mean then, yes,
I do that to map grefs
> see public/grant_table.h's explanation of GNTMAP_host_map used
> with out without GNTMAP_contains_pte.
I know about these options and according to [1] I can use
option to map with "host virtual address", e.g. without 
GNTMAP_contains_pte flag, because GNTMAP_contains_pte requires me to 
provide machine address which I don't want.
>   Other mapping attempts
> have to be refused, or else the accounting done by the grant table
> code would be undermined.
So, either I didn't understand what you mean or was not clear
to explain that I see no problem while mapping grefs with
gnttab_map_refs, but see the problem when vm_insert_page
is called to mmap the pages into user-space (vm_insert_page
internally does set_pte_at which it silently fails, but kernel
knows nothing about that because no error reported [2]).
> Jan
Thank you,
Oleksandr

[1] 
http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/public/grant_table.h;h=e5f04ec57c5bd590b6c81799ee01028c3512aacf;hb=refs/heads/master#l331
[2] http://lxr.free-electrons.com/source/mm/memory.c#L1569

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