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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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,
see public/grant_table.h's explanation of GNTMAP_host_map used
with out without GNTMAP_contains_pte. Other mapping attempts
have to be refused, or else the accounting done by the grant table
code would be undermined.

Jan


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