Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2015-07-28 15:06, Michael S. Tsirkin wrote:
> On Tue, Jul 28, 2015 at 02:46:20PM +0200, Paolo Bonzini wrote:
>>
>>
>> On 28/07/2015 12:12, Benjamin Herrenschmidt wrote:
>>>>> That is an experimental feature (it's x-iommu), so it can change.
>>>>>
>>>>> The plan was:
>>>>>
>>>>> - for PPC, virtio never honors IOMMU
>>>>>
>>>>> - for non-PPC, either have virtio always honor IOMMU, or enforce that
>>>>> virtio is not under IOMMU.
>>>>>
>>> I dislike having PPC special cased.
>>>
>>> In fact, today x86 guests also assume that virtio bypasses IOMMU I
>>> believe. In fact *all* guests do.
>>
>> This doesn't matter much, since the only guests that implement an IOMMU
>> in QEMU are (afaik) PPC and x86, and x86 does not yet promise any kind
>> of stability.
> 
> Hmm I think Jan (cc) said it was already used out there.

Yes, no known issues with vt-d emulation for almost a year now. Error
reporting could be improved, and interrupt remapping is still missing,
but those are minor issues in this context.

In my testing setups, I also have virtio devices in use, passed through
to an L2 guest, but only in 1:1 mapping so that their broken IOMMU
support causes no practical problems.

Jan

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