Re: [Xen-devel] Having a DOM-U guest with 1:1 mapping in the second stage MMU.

[Julien Grall <julien@xxxxxxx>]

On 14/01/2020 21:39, Jorge Pereira wrote:
> Hi Guys,
Hello,

> I’m currently using XEN in order to run side-by-side a DOM-0 with a 
> DOM-U guest. My use-case scenario requires in the DOM-U direct access to 
> some dma-capable devices such ethernet and some GPUs.
> Since our target platform (i.MX8MM) does not support IOMMU, we can’t 
> assign dma-capable devices to the DOM-U guest because XEN does not 
> create 1:1 mapping for that guest in the 2^nd stage MMU. So, 
> guest-virtual addresses are different than the physical ones.
Bear in mind this setup is going to be insecure unless you have another 
way to prevent your passthrough-ed device to access memory it should not 
(e.g an MPU).
> Is it possible to have 1:1 mapping for DOM-U guests?
It is not possible at the moment. There are been various effort to try 
to do it, but I have always push back as this is actively defeating the 
purposing of an hypervisor.
This would be a different story if we had support for MPU in Xen.

> If not, I’m 
> interested to know what would be the estimated effort to support this 
> feature?
I think you have someone else in NXP looking at 1:1 mapping for Xen (in 
CC). I provided to Andrei some tips how to get 1:1 mapping for DomU 
using dom0less in December (see [1]). So you may want to sync-up with 
him here.
If you are looking at 1:1 DomU using xl, then it is going to require 
more work as the hypercall allocating memory is based on guest frame 
number. There was a thread on the ML a few years ago, I can try to dig 
it down if you are interested.
Cheers,

[1] 
https://lists.xenproject.org/archives/html/xen-devel/2019-12/msg01364.html
> Thanks in advance,
>
> Cheers,
>
> Jorge
>
>
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--
Julien Grall

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