Re: x86: mapping from Dom0 to DomU

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 04/24/2017 09:43 AM, Jan Beulich wrote:
>>>> On 24.04.17 at 08:10, <andr2000@gmail.com> wrote:
>> On 04/21/2017 06:55 PM, Jan Beulich wrote:
>>>>>> On 21.04.17 at 15:04, <andr2000@gmail.com> 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
>
thank you,
it seems like I'll have to duplicate code from gntdev [1],
so my use-case also works on x86, not only ARM

[1] http://lxr.free-electrons.com/source/drivers/xen/gntdev.c#L981

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