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    SubjectRe: [PATCH V2 RFC] fixup! virtio: convert to use DMA api
    Add Stefano and Anthony

    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@redhat.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)
    >
    > hwaddr virtio_queue_get_desc_addr(VirtIODevice *vdev, int n);
    > hwaddr virtio_queue_get_avail_addr(VirtIODevice *vdev, int n);
    > diff --git a/include/standard-headers/linux/virtio_config.h b/include/standard-headers/linux/virtio_config.h
    > index bcc445b..3fcfbb1 100644
    > --- a/include/standard-headers/linux/virtio_config.h
    > +++ b/include/standard-headers/linux/virtio_config.h
    > @@ -61,4 +61,6 @@
    > /* v1.0 compliant. */
    > #define VIRTIO_F_VERSION_1 32
    >
    > +/* Do not bypass the IOMMU (if configured) */
    > +#define VIRTIO_F_IOMMU_PLATFORM 34
    > #endif /* _LINUX_VIRTIO_CONFIG_H */
    > --
    > MST

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