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

Benjamin Herrenschmidt
2014-09-03T08:05:36Z

On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
>
> IOMMU support for x86 is going to go in this week.

But won't that break virtio on x86 ? Or will virtio continue bypassing
it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't call
the dma mappings ops.

> However, it is and likely will remain niche enough that I don't really
> care about performance loss from IOMMU support. If you enable it, you
> want it.
>
> So from the QEMU point of view we can simply add the direct-ram-access
> property, and have the pseries machine turn it on by default (while
> other machines can leave it off by default---they have no IOMMU and
> thus no performance cost).

Well, it's only for virtio and should be on by default on x86 as well if
an iommu is installed no ?

Cheers,
Ben.