Re: [Xen-devel] [PATCH V4] xen/arm: domain_build: allocate lowmem for dom0 as much as possible

[Julien Grall <julien.grall@xxxxxxx>]

On 11/11/16 14:24, Andrii Anisov wrote:
> Hello Julien,
>
> Please see my comments below:
>
> > From my understanding of what you say, the problem is not because domU is using 
> > memory above 4GB but the fact that >the backend driver does not take the right 
> > decision
> Yep, the problem could be treated in such a way.
>
> > (e.g using bounce buffer when required).
> I suppose unacceptable performance drop for such kind of solution.
Could you define unacceptable performance drop? Have you tried to 
measure what would be the impact?
> An alternative here could be reverting of the FE-BE interaction scheme
> in a following way: BE side domain provides buffers and maps them to
> the FE side domain. Some time ago we estimated this approach as huge
> architecture change and enormous implementation efforts.
You could also exhaust the memory of the backend domain.

> Also it does
> answer to the next question:
>
> > The guest should be IPA agnostic and not care how the physical device is 
> > working when using PV drivers. So for me,
> > this should be fixed in the DOM0 OS.
> Do you consider driver domain guests?
The main point of driver domain is isolating a device/driver in a 
specific guest. For that you need an SMMU to secure the device, which 
would also solve the issue with 32-bit DMA-capable device.
So why would you want to do driver domain without an SMMU present?

Regards,

--
Julien Grall

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