Re: x86: mapping from Dom0 to DomU

[Oleksandr Andrushchenko <andr2000@gmail.com>]

Hi, Jan!

On 04/21/2017 06:55 PM, Jan Beulich wrote:
>>>> On 21.04.17 at 15:04, <andr2000@gmail.com> 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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