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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Jul 28, 2015 6:11 AM, "Jan Kiszka" <jan.kiszka@xxxxxxxxxxx> wrote:
>
> 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.
>

How are you getting 1:1 to work?  Is it something that L0 QEMU can
advertise to L1?  If so, can we just do that unconditionally, which
would make my patch work?

I have no objection to 1:1 devices in general.  It's only devices that
the PCI code on the guest identifies as not 1:1 but that are
nonetheless 1:1 that cause problems.

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