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

Konrad Rzeszutek Wilk
2014-09-02T20:56:26Z

On Wed, Sep 03, 2014 at 06:53:33AM +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2014-09-01 at 22:55 -0700, Andy Lutomirski wrote:
> >
> > On x86, at least, I doubt that we'll ever see a physically addressed
> > PCI virtio device for which ACPI advertises an IOMMU, since any sane
> > hypervisor will just not advertise an IOMMU for the virtio device.
> > But are there arm64 or PPC guests that use virtio_pci, that have
> > IOMMUs, and that will malfunction if the virtio_pci driver ends up
> > using the IOMMU? I certainly hope not, since these systems might be
> > very hard-pressed to work right if someone plugged in a physical
> > virtio-speaking PCI device.
>
> It will definitely not work on ppc64. We always have IOMMUs on pseries,
> all PCI busses do, and because it's a paravirtualized environment,
> napping/unmapping pages means hypercalls -> expensive.
>
> But our virtio implementation bypasses it in qemu, so if virtio-pci
> starts using the DMA mapping API without changing the DMA ops under the
> hood, it will break for us.

What is the default dma_ops that the Linux guests start with as
guests under ppc64?

Thanks!
>
> Cheers,
> Ben.
>
>