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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Thu, 23 Oct 2014 07:34:16 +1100
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Subject: Re: [Qemu-devel] [PATCH RFC 00/11] qemu: towards virtio-1 host
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To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: thuth@linux.vnet.ibm.com, kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel@nongnu.org, Andy Lutomirski <luto@amacapital.net>, Cornelia Huck <cornelia.huck@de.ibm.com>, virtualization@lists.linux-foundation.org

On Wed, 2014-10-22 at 16:17 +0200, Jan Kiszka wrote:
> I thought about this again, and I'm not sure anymore if we can use
> ACPI
> to "black-list" the incompatible virtio devices. Reason: hotplug. To
> my
> understanding, the ACPI DRHD tables won't change during runtime when a
> device shows up or disappears. We would have to isolate virtio devices
> from the rest of the system by using separate buses for it (and avoid
> listing those in any DRHD table) and enforce that they only get
> plugged
> into those buses. I suppose that is not desirable.
> 
> Maybe it's better to fix virtio /wrt IOMMUs.

I always go back to my initial proposal which is to define that current
virtio always bypass any iommu (which is what it does really) and have
it expose via a new capability if that isn't the case. That means fixing
that Xen thingy to allow qemu to know what to expose I assume but that
seems to be the less bad approach.

Cheers,
Ben.