From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [RFC 7/11] virtio_pci: new, capability-aware driver.
Date: Wed, 11 Jan 2012 20:34:41 +0200
Message-ID: <20120111183440.GC23753@redhat.com>
References: <871us0om2t.fsf@rustcorp.com.au> <20111220113718.GF3913@redhat.com>
	<878vm6daqy.fsf@rustcorp.com.au>
	<20120110170334.GA18404@redhat.com>
	<8762gj6q5r.fsf@rustcorp.com.au>
	<CAJSP0QU_viNks2jv003c8+gYWFkyW=FvZpU5w6wyroeEyFkXsA@mail.gmail.com>
	<1326273019.23910.118.camel@pasglop>
	<CAJSP0QWMBhsg9ExmTsDG7d32p=7tokCPpm5Z0DMAm2_ADvvb+g@mail.gmail.com>
	<20120111153954.GC20570@redhat.com>
	<CAJSP0QVm2f+A8hFa+tx3LRcXa_oe8mn4=DhrGMnozVB8_K4OxQ@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
Return-path: <virtualization-bounces@lists.linux-foundation.org>
Content-Disposition: inline
In-Reply-To: <CAJSP0QVm2f+A8hFa+tx3LRcXa_oe8mn4=DhrGMnozVB8_K4OxQ@mail.gmail.com>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/virtualization/>
List-Post: <mailto:virtualization@lists.linux-foundation.org>
List-Help: <mailto:virtualization-request@lists.linux-foundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=subscribe>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org
To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Pawel Moll <pawel.moll@arm.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, virtualization <virtualization@lists.linux-foundation.org>, Christian Borntraeger <borntraeger@de.ibm.com>, Sasha Levin <levinsasha928@gmail.com>
List-Id: virtualization@lists.linuxfoundation.org

On Wed, Jan 11, 2012 at 05:21:53PM +0000, Stefan Hajnoczi wrote:
> On Wed, Jan 11, 2012 at 3:39 PM, Michael S. Tsirkin <mst@redhat.com> wrot=
e:
> > On Wed, Jan 11, 2012 at 02:28:48PM +0000, Stefan Hajnoczi wrote:
> >> On Wed, Jan 11, 2012 at 9:10 AM, Benjamin Herrenschmidt
> >> <benh@kernel.crashing.org> wrote:
> >> > On Wed, 2012-01-11 at 08:47 +0000, Stefan Hajnoczi wrote:
> >> >>
> >> >> This is also an opportunity to stop using CPU physical addresses in
> >> >> the ring and instead perform DMA like a normal PCI device (use bus
> >> >> addresses).
> >> >
> >> > Euh why ?
> >>
> >> Because it's a paravirt hack that ends up hitting corner cases. =A0It's
> >> not possible to do virtio-pci passthrough under nested virtualization
> >> unless we use an IOMMU. =A0Imagine passing virtio-net from L0 into the
> >> L2 guest (i.e. PCI-passthrough). =A0If virtio-pci is really "PCI" this
> >> should be possible but it's not when we use physical addresses instead
> >> of bus addresses.
> >>
> >> Stefan
> >
> > It won't be hard to show siginificant performance regression if
> > we do this. Hard to justify for something as niche as nested virt.
> =

> For x86 this should be mostly a nop.

Maybe it should, but AFAIK it isn't.

> For ppc and SPARC architectures maybe you're right.  I still think
> it's a design flaw because if virtio v2 doesn't use bus addresses then
> it will simply not be possible to do passthrough for nested virt and
> other cases we haven't hit yet.
> =

> Stefan

virtio-pci does not implement things like SRIOV or FLR so it
won't work anyway.

If we ever fix this, and if we really want to pass through a virtio
device (why?) using an emulated iommu seems silly - we probably want a
PV IOMMU as well.

-- =

MST