Re: [Qemu-block] [PATCH for 2.8 02/11] virtio: convert to use DMA api

[Cornelia Huck]

On Tue, 30 Aug 2016 11:06:50 +0800
Jason Wang <address@hidden> wrote:

> Currently, all virtio devices bypass IOMMU completely. This is because
> address_space_memory is assumed and used during DMA emulation. This
> patch converts the virtio core API to use DMA API. This idea is
> 
> - introducing a new transport specific helper to query the dma address
>   space. (only pci version is implemented).
> - query and use this address space during virtio device guest memory
>   accessing when iommu platform (VIRTIO_F_IOMMU_PLATFORM) was enabled
>   for this device.
> 
> Cc: Michael S. Tsirkin <address@hidden>
> Cc: Stefan Hajnoczi <address@hidden>
> Cc: Kevin Wolf <address@hidden>
> Cc: Amit Shah <address@hidden>
> Cc: Paolo Bonzini <address@hidden>
> Cc: address@hidden
> Signed-off-by: Jason Wang <address@hidden>
> ---
>  hw/block/virtio-blk.c             |  2 +-
>  hw/char/virtio-serial-bus.c       |  3 +-
>  hw/scsi/virtio-scsi.c             |  4 ++-
>  hw/virtio/virtio-pci.c            | 14 +++++++++
>  hw/virtio/virtio.c                | 62 
> ++++++++++++++++++++++++---------------
>  include/hw/virtio/virtio-access.h | 43 ++++++++++++++++++++-------
>  include/hw/virtio/virtio-bus.h    |  1 +
>  include/hw/virtio/virtio.h        |  8 +++--
>  8 files changed, 98 insertions(+), 39 deletions(-)
> 

> diff --git a/include/hw/virtio/virtio-access.h 
> b/include/hw/virtio/virtio-access.h
> index 440b455..4071dad 100644
> --- a/include/hw/virtio/virtio-access.h
> +++ b/include/hw/virtio/virtio-access.h
> @@ -17,12 +17,25 @@
>  #define QEMU_VIRTIO_ACCESS_H
> 
>  #include "hw/virtio/virtio.h"
> +#include "hw/virtio/virtio-bus.h"
>  #include "exec/address-spaces.h"
> 
>  #if defined(TARGET_PPC64) || defined(TARGET_ARM)
>  #define LEGACY_VIRTIO_IS_BIENDIAN 1
>  #endif
> 
> +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 (virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM) &&
> +        k->get_dma_as) {
> +        return k->get_dma_as(qbus->parent);
> +    }
> +    return &address_space_memory;
> +}

One thing I'm a bit worried about is that we're introducing a check
even if we know that the device will never support
VIRTIO_F_IOMMU_PLATFORM (i.e. virtio-ccw). The qom incantations will
add cycles to any invocation of this.

Is the address space likely to change during device lifetime? Can we
cache it in some way?