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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.04.17 at 08:10, <andr2000@xxxxxxxxx> wrote:
> On 04/21/2017 06:55 PM, Jan Beulich wrote:
>>>>> On 21.04.17 at 15:04, <andr2000@xxxxxxxxx> wrote:
>>> 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
>>>
>>> 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]).

Indeed you seem to have misunderstood: _All_ mappings of the
granted page need to be done using the grant table hypercalls,
not just the initial one. set_pte() establishes a second mapping,
and that does not use the grant table op.

Jan


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