linux-kernel - Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api

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Date:	Fri, 22 Apr 2016 10:33:02 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Stefan Hajnoczi <stefanha@...hat.com>,
	Wei Liu <wei.liu2@...rix.com>, kvm@...r.kernel.org,
	qemu-block@...gnu.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	qemu-devel@...gnu.org, peterx@...hat.com,
	linux-kernel@...r.kernel.org, Amit Shah <amit.shah@...hat.com>,
	Andy Lutomirski <luto@...nel.org>, pbonzini@...hat.com,
	virtualization@...ts.linux-foundation.org,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api

On Thu, Apr 21, 2016 at 06:11:40PM +0300, Michael S. Tsirkin wrote:
> On Thu, Apr 21, 2016 at 03:56:53PM +0100, Stefan Hajnoczi wrote:
> > On Thu, Apr 21, 2016 at 04:43:45PM +0300, Michael S. Tsirkin wrote:
> > > This adds a flag to enable/disable bypassing the IOMMU by
> > > virtio devices.
> > > 
> > > This is on top of patch
> > > http://article.gmane.org/gmane.comp.emulators.qemu/403467
> > > virtio: convert to use DMA api
> > > 
> > > Tested with patchset
> > > http://article.gmane.org/gmane.linux.kernel.virtualization/27545
> > > virtio-pci: iommu support (note: bit number has been kept at 34
> > > intentionally to match posted guest code. a non-RFC version will
> > > renumber bits to be contigious).
> > > 
> > > changes from v1:
> > >     drop PASSTHROUGH flag
> > > 
> > > The interaction between virtio and DMA API is messy.
> > > 
> > > On most systems with virtio, physical addresses match bus addresses,
> > > and it doesn't particularly matter whether we use the DMA API.
> > > 
> > > On some systems, including Xen and any system with a physical device
> > > that speaks virtio behind a physical IOMMU, we must use the DMA API
> > > for virtio DMA to work at all.
> > > 
> > > Add a feature bit to detect that: VIRTIO_F_IOMMU_PLATFORM.
> > > 
> > > If not there, we preserve historic behavior and bypass the DMA
> > > API unless within Xen guest. This is actually required for
> > > systems, including SPARC and PPC64, where virtio-pci devices are
> > > enumerated as though they are behind an IOMMU, but the virtio host
> > > ignores the IOMMU, so we must either pretend that the IOMMU isn't
> > > there or somehow map everything as the identity.
> > > 
> > > Re: non-virtio devices.
> > > 
> > > It turns out that on old QEMU hosts, only emulated devices which were
> > > part of QEMU use the IOMMU.  Should we want to bypass the IOMMU for such
> > > devices *only*, it would be rather easy to detect them by looking at
> > > subsystem vendor and device ID. Thus, no new interfaces are required
> > > except for virtio which always uses the same subsystem vendor and device ID.
> > > 
> > > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > > ---
> > >  include/hw/virtio/virtio-access.h              | 3 ++-
> > >  include/hw/virtio/virtio.h                     | 4 +++-
> > >  include/standard-headers/linux/virtio_config.h | 2 ++
> > >  3 files changed, 7 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/include/hw/virtio/virtio-access.h b/include/hw/virtio/virtio-access.h
> > > index 967cc75..bb6f34e 100644
> > > --- a/include/hw/virtio/virtio-access.h
> > > +++ b/include/hw/virtio/virtio-access.h
> > > @@ -23,7 +23,8 @@ static inline AddressSpace *virtio_get_dma_as(VirtIODevice *vdev)
> > >      BusState *qbus = qdev_get_parent_bus(DEVICE(vdev));
> > >      VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
> > >  
> > > -    if (k->get_dma_as) {
> > > +    if (virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM) &&
> > > +        k->get_dma_as) {
> > >          return k->get_dma_as(qbus->parent);
> > >      }
> > >      return &address_space_memory;
> > > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > > index b12faa9..44f3788 100644
> > > --- a/include/hw/virtio/virtio.h
> > > +++ b/include/hw/virtio/virtio.h
> > > @@ -228,7 +228,9 @@ typedef struct VirtIORNGConf VirtIORNGConf;
> > >      DEFINE_PROP_BIT64("notify_on_empty", _state, _field,  \
> > >                        VIRTIO_F_NOTIFY_ON_EMPTY, true), \
> > >      DEFINE_PROP_BIT64("any_layout", _state, _field, \
> > > -                      VIRTIO_F_ANY_LAYOUT, true)
> > > +                      VIRTIO_F_ANY_LAYOUT, true), \
> > > +    DEFINE_PROP_BIT64("iommu_platform", _state, _field, \
> > > +                      VIRTIO_F_IOMMU_PLATFORM, false)
> > 
> > Looks like the impact of this patch is that users who relied on
> > k->get_dma_as today may now have to explicitly add iommu_platform=on.
> > Are there any such users (e.g. Xen)?
> 
> No because upstream this is ignored. This is an incremental patch
> on top of Jason's one.
> 
> > Instead of breaking the command-line for these users you could invert
> > the flag's meaning ("iommu_bypass=on") and set it in the SPARC/PPC
> > machine types.
> > 
> > Stefan
> 
> I hope I made it clear that there are no such users.

Okay, thanks!

Reviewed-by: Stefan Hajnoczi <stefanha@...hat.com>

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