Re: [PATCH v3 0/3] map grant refs at pfn = mfn

[Julien Grall <julien.grall@linaro.org>]

On 01/08/14 17:16, Thomas Leonard wrote:
> On 1 August 2014 16:13, Stefano Stabellini
> <stefano.stabellini@eu.citrix.com> wrote:
>> On Fri, 1 Aug 2014, Thomas Leonard wrote:
>>> On 24/07/14 14:30, Stefano Stabellini wrote:
>>>> Hi all,
>>>> this patch series introduces a second p2m mapping of grant reference on
>>>> ARM at guest physical address == machine address of the grant ref.  It
>>>> is safe because dom0 is already mapped 1:1. We export
>>>> XENFEAT_grant_map_identity to signal the guest that this second p2m
>>>> mapping is
>>>> available.
>>>>
>>>> One reason for wanting the second p2m mapping is to avoid tracking mfn
>>>> to pfn mappings in the guest kernel. Since the same mfn can be granted
>>>> multiple times to the backend, finding the right pfn corresponding to a
>>>> given mfn can be difficult and expensive. Providing a second mapping at
>>>> a known address allow the kernel to access the page without knowing the
>>>> pfn.
>>>
>>> Is there a version of these patches for Xen 4.4 that I can test? The
>>> restriction on duplicate pages is causing trouble for networking on Mirage too
>>> (http://roscidus.com/blog/blog/2014/07/28/my-first-unikernel/#tcp-retransmissions).
>>
>> The backport is non-trivial because
>> 552710b388630dfa461932940a998e120c42277d is missing from 4.4,
>> nonetheless it wasn't too hard to port:
>>
>>    git://xenbits.xen.org/people/sstabellini/xen-unstable.git grant_map_identity_4.4
>
> Thanks. I merged it with the stable-4.4 branch (as that has some
> useful fixes too), but it crashed for me when I started my Mirage
> guest:

You have to drop the BUG_ON line 729.

Regards,

-- 
Julien Grall