Re: [Xen-devel] [PATCH] xen: grant-table: Check truncation when giving access to a frame

[Julien Grall <julien.grall@arm.com>]

On 13/06/16 13:41, Julien Grall wrote:
> Hello Paul,
>
> On 13/06/16 13:12, Paul Durrant wrote:
>>> -----Original Message-----
>>> From: Xen-devel [mailto:xen-devel-bounces@lists.xen.org] On Behalf Of
>>> Julien Grall
>>> Sent: 13 June 2016 11:51
>>> To: boris.ostrovsky@oracle.com; David Vrabel; jgross@suse.com;
>>> sstabellini@kernel.org; konrad.wilk@oracle.com
>>> Cc: steve.capper@arm.com; Andrew Cooper; linux-kernel@vger.kernel.org;
>>> xen-devel@lists.xen.org; Julien Grall; JBeulich@suse.com
>>> Subject: [Xen-devel] [PATCH] xen: grant-table: Check truncation when
>>> giving
>>> access to a frame
>>>
>>> The version 1 of the grant-table protocol only supports frame encoded on
>>> 32-bit.
>>>
>>> When the platform is supporting 48-bit physical address, the frame will
>>> be encoded on 36-bit which will lead a truncation and give access to
>>> the wrong frame.
>>>
>>> On ARM Xen will always allow the guest to use all the physical address,
>>> although today the RAM is always located under 40-bits (see
>>> xen/include/public/arch-arm.h).
>>>
>>> Add a truncation check in gnttab_update_entry_v1 to prevent the guest to
>>> give access to the wrong frame.
>>>
>>> Signed-off-by: Julien Grall <julien.grall@arm.com>
>>>
>>> ---
>>>      This is limiting us to a 44-bit address space whilst ARM can
>>> support
>>>      up to 48-bit today. This number of bit will increase to 52-bit in
>>>      upcoming processors [1].
>>>
>>>      It might be good to start thinking to extend the version 1 of the
>>>      protocol to use 64-bit frame number.
>>
>> ...or simply use version 2 of the protocol.
>
> On another mail [1], you said that "[v2] didn't scale it became
> bottle-necked on dom0's grant table size,...".
>
> So it looks like to me that version 2 is the wrong way to go.
> The performance should stay the same whether the platform support
> 40-bit, 44-bit, 48-bit, 52-bit address space.

I forgot the link.

[1] 
http://lists.xenproject.org/archives/html/xen-devel/2016-06/msg01606.html

-- 
Julien Grall