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

Paolo Bonzini
2014-09-03T07:47:07Z

Il 03/09/2014 02:25, Benjamin Herrenschmidt ha scritto:
> > But there aren't any ACPI systems with both virtio-pci and IOMMUs,
> > right? So we could say that, henceforth, ACPI systems must declare
> > whether virtio-pci devices live behind IOMMUs without breaking
> > backward compatibility.
>
> I don't know for sure whether that's the case and whether we can rely on
> that not happening, we'll need x86 folks opinion here.

IOMMU support for x86 is going to go in this week.

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).

Paolo