From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH RFC 00/11] qemu: towards virtio-1 host support
Date: Thu, 23 Oct 2014 10:12:51 +0300
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: thuth@linux.vnet.ibm.com, kvm@vger.kernel.org, Rusty Russell <rusty@au1.ibm.com>, Jan Kiszka <jan.kiszka@siemens.com>, qemu-devel@nongnu.org, Andy Lutomirski <luto@amacapital.net>, virtualization@lists.linux-foundation.org
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On Thu, Oct 23, 2014 at 07:34:16AM +1100, Benjamin Herrenschmidt wrote:
> 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.
> 

OK so how does this work?
If you want to run an existing guest, you use the old device.
And presumably you blacklist virtio for nested virt then,
unless a new capability is present?


-- 
MST