Re: [PATCH 0/9] Refine virtio mapping API

[Jason Wang]

On Tue, Jul 1, 2025 at 3:04 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Tue, Jul 01, 2025 at 09:13:52AM +0800, Jason Wang wrote:
> > Hi all:
> >
> > Virtio used to be coupled with DMA API. This works fine for the device
> > that do real DMA but not the others. For example, VDUSE nees to craft
> > with DMA API in order to let the virtio-vdpa driver to work.
> >
> > This series tries to solve this issue by introducing the mapping API
> > in the virtio core. So transport like vDPA can implement their own
> > mapping logic without the need to hack with DMA API. The mapping API
> > are abstracted with a new map operations in order to be re-used by
> > transprot or device. So device like VDUSE can implement its own
> > mapping loigc.
> >
> > Please review.
> >
> > Thanks
>
> Cost of all this extra indirection? Especially on systems with
> software spectre mitigations/retpoline enabled.

Actually not, it doesn't change how things work for the device that
does DMA already like:

If device has its specific mapping ops
        go for device specific mapping ops
else
        go for DMA API

VDUSE is the only user now, and extra indirection has been used for
VDUSE even without this series (via abusing DMA API). This series
switch from:

virtio core -> DMA API -> VDUSE DMA API -> iova domain ops

to

virtio core -> virtio map ops -> VDUSE map ops -> iova domain ops

Thanks



>
> > Jason Wang (9):
> >   virtio_ring: constify virtqueue pointer for DMA helpers
> >   virtio_ring: switch to use dma_{map|unmap}_page()
> >   virtio: rename dma helpers
> >   virtio: rename dma_dev to map_token
> >   virtio_ring: rename dma_handle to map_handle
> >   virtio: introduce map ops in virtio core
> >   vdpa: rename dma_dev to map_token
> >   vdpa: introduce map ops
> >   vduse: switch to use virtio map API instead of DMA API
> >
> >  drivers/net/virtio_net.c                 |  32 +-
> >  drivers/vdpa/alibaba/eni_vdpa.c          |   5 +-
> >  drivers/vdpa/ifcvf/ifcvf_main.c          |   5 +-
> >  drivers/vdpa/octeon_ep/octep_vdpa_main.c |   6 +-
> >  drivers/vdpa/pds/vdpa_dev.c              |   3 +-
> >  drivers/vdpa/solidrun/snet_main.c        |   4 +-
> >  drivers/vdpa/vdpa.c                      |   5 +-
> >  drivers/vdpa/vdpa_sim/vdpa_sim.c         |   4 +-
> >  drivers/vdpa/vdpa_user/iova_domain.c     |   8 +-
> >  drivers/vdpa/vdpa_user/iova_domain.h     |   5 +-
> >  drivers/vdpa/vdpa_user/vduse_dev.c       |  34 +-
> >  drivers/vdpa/virtio_pci/vp_vdpa.c        |   5 +-
> >  drivers/vhost/vdpa.c                     |  11 +-
> >  drivers/virtio/virtio_ring.c             | 440 ++++++++++++++---------
> >  drivers/virtio/virtio_vdpa.c             |  15 +-
> >  include/linux/vdpa.h                     |  22 +-
> >  include/linux/virtio.h                   |  36 +-
> >  include/linux/virtio_config.h            |  68 ++++
> >  include/linux/virtio_ring.h              |   6 +-
> >  19 files changed, 476 insertions(+), 238 deletions(-)
> >
> > --
> > 2.34.1
>