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

[Stefan Hajnoczi]

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 <address@hidden>
> ---
>  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)?

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

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