Re: [Xen-devel] [early RFC] ARM PCI Passthrough design document

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Stefano,

On 02/13/2017 07:59 PM, Stefano Stabellini wrote:
> On Mon, 13 Feb 2017, Julien Grall wrote:
>> Hi Stefano,
>>
>> On 10/02/17 01:01, Stefano Stabellini wrote:
>>> On Fri, 3 Feb 2017, Edgar E. Iglesias wrote:
>>>> A possible hack could be to allocate a chunk of DDR dedicated for PCI DMA.
>>>> PCI DMA devs could be locked in to only be able to access this mem + MSI
>>>> doorbell.
>>>> Guests can still screw each other up but at least it becomes harder to
>>>> read/write directly from each others OS memory.
>>>> It may not be worth the effort though....
>>>
>>> Actually, we do have the swiotlb in Dom0, which can be used to bounce
>>> DMA requests over a buffer that has been previously setup to be DMA safe
>>> using an hypercall. That is how the swiotlb is used on x86. On ARM it is
>>> used to issue cache flushes via hypercall, but it could be adapted to do
>>> both. It would degrade performance, due to the additional memcpy, but it
>>> would work, I believe.
>>
>> A while ago, Globallogic suggested to use direct memory mapping for the guest
>> to allow guest using DMA on platform not supporting SMMU.
>>
>> I believe we can use the same trick on platform where SMMUs can not
>> distinguish PCI devices.
>
> Yes, that would work, but only on platforms with a very limited number
> of guests. However, it might still be a very common use-case on a
> platform such as the Zynq MPSoC.

Can you explain why you think this could only work with limited number
of guests?

Cheers,

-- 
Julien Grall

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