Re: [PATCH RFC 1/4] vdpa: introduce .reset_map operation callback

[Si-Wei Liu]

On 8/15/2023 6:55 PM, Jason Wang wrote:
> On Wed, Aug 16, 2023 at 3:49 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:
> > On 8/14/2023 7:21 PM, Jason Wang wrote:
> > > On Tue, Aug 15, 2023 at 9:46 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote:
> > > > Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
> > > > ---
> > > >    include/linux/vdpa.h | 7 +++++++
> > > >    1 file changed, 7 insertions(+)
> > > >
> > > > diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
> > > > index db1b0ea..3a3878d 100644
> > > > --- a/include/linux/vdpa.h
> > > > +++ b/include/linux/vdpa.h
> > > > @@ -314,6 +314,12 @@ struct vdpa_map_file {
> > > >     *                             @iova: iova to be unmapped
> > > >     *                             @size: size of the area
> > > >     *                             Returns integer: success (0) or error (< 0)
> > > > + * @reset_map:                 Reset device memory mapping (optional)
> > > > + *                             Needed for device that using device
> > > > + *                             specific DMA translation (on-chip IOMMU)
> > > This exposes the device internal to the upper layer which is not optimal.
> > Not sure what does it mean by "device internal", but this op callback
> > just follows existing convention to describe what vdpa parent this API
> > targets.
> I meant the bus tries to hide the differences among vendors. So it
> needs to hide on-chip IOMMU stuff to the upper layer.
>
> We can expose two dimensional IO mappings models but it looks like
> over engineering for this issue. More below.
>
> >    * @set_map:                    Set device memory mapping (optional)
> >    *                              Needed for device that using device
> >    *                              specific DMA translation (on-chip IOMMU)
> > :
> > :
> >    * @dma_map:                    Map an area of PA to IOVA (optional)
> >    *                              Needed for device that using device
> >    *                              specific DMA translation (on-chip IOMMU)
> >    *                              and preferring incremental map.
> > :
> > :
> >    * @dma_unmap:                  Unmap an area of IOVA (optional but
> >    *                              must be implemented with dma_map)
> >    *                              Needed for device that using device
> >    *                              specific DMA translation (on-chip IOMMU)
> >    *                              and preferring incremental unmap.
> >
> >
> > > Btw, what's the difference between this and a simple
> > >
> > > set_map(NULL)?
> > I don't think parent drivers support this today - they can accept
> > non-NULL iotlb containing empty map entry, but not a NULL iotlb. The
> > behavior is undefined or it even causes panic when a NULL iotlb is
> > passed in.
> We can do this simple change if it can work.
If we go with setting up 1:1 DMA mapping at virtio-vdpa .probe() and 
tearing it down at .release(), perhaps set_map(NULL) is not sufficient.
> >   Further this doesn't work with .dma_map parent drivers.
> Probably, but I'd remove dma_map as it doesn't have any real users
> except for the simulator.
OK, at a point there was suggestion to get this incremental API extended 
to support batching to be in par with or even replace .set_map, not sure 
if it's too soon to conclude. But I'm okay with the removal if need be.
> > The reason why a new op is needed or better is because it allows
> > userspace to tell apart different reset behavior from the older kernel
> > (via the F_IOTLB_PERSIST feature bit in patch 4), while this behavior
> > could vary between parent drivers.
> I'm ok with a new feature flag, but we need to first seek a way to
> reuse the existing API.
A feature flag is needed anyway. I'm fine with reusing but guess I'd 
want to converge on the direction first.

Thanks,
-Siwei
> Thanks
>
> > Regards,
> > -Siwei
> >
> > > Thanks
> > >
> > > > + *                             @vdev: vdpa device
> > > > + *                             @asid: address space identifier
> > > > + *                             Returns integer: success (0) or error (< 0)
> > > >     * @get_vq_dma_dev:            Get the dma device for a specific
> > > >     *                             virtqueue (optional)
> > > >     *                             @vdev: vdpa device
> > > > @@ -390,6 +396,7 @@ struct vdpa_config_ops {
> > > >                          u64 iova, u64 size, u64 pa, u32 perm, void *opaque);
> > > >           int (*dma_unmap)(struct vdpa_device *vdev, unsigned int asid,
> > > >                            u64 iova, u64 size);
> > > > +       int (*reset_map)(struct vdpa_device *vdev, unsigned int asid);
> > > >           int (*set_group_asid)(struct vdpa_device *vdev, unsigned int group,
> > > >                                 unsigned int asid);
> > > >           struct device *(*get_vq_dma_dev)(struct vdpa_device *vdev, u16 idx);
> > > > --
> > > > 1.8.3.1