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From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH v3 0/3] virtio DMA API core stuff
Date: Mon, 23 Nov 2015 09:56:56 +0200
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To: David Woodhouse <dwmw2@infradead.org>
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On Sun, Nov 22, 2015 at 10:21:34PM -0000, David Woodhouse wrote:
> 
> 
> > There's that, and there's an "I care about security, but
> > do not want to burn up cycles on fake protections that
> > do not work" case.
> 
> It would seem to make most sense for this use case simply *not* to expose
> virtio devices to guests as being behind an IOMMU at all. Sure, there are
> esoteric use cases where the guest actually nests and runs further guests
> inside itself and wants to pass through the virtio devices from the real
> hardware host. But presumably those configurations will have multiple
> virtio devices assigned by the host anyway,  and further tweaking the
> configuration to put them behind an IOMMU shouldn't be hard.

Unfortunately it's a no-go: this breaks the much less esoteric usecase
of DPDK: using virtio devices with userspace drivers.

Well - not breaks as such as this doesn't currently work,
but this approach would prevent us from making it work.

> 
> -- 
> dwmw2