Re: [PATCH 5/5] virtio: Add bounce DMA ops

Michael S. Tsirkin
2020-04-28T20:41:16Z

On Tue, Apr 28, 2020 at 11:19:52PM +0530, Srivatsa Vaddagiri wrote:
> * Michael S. Tsirkin <mst@redhat.com> [2020-04-28 12:17:57]:
>
> > Okay, but how is all this virtio specific? For example, why not allow
> > separate swiotlbs for any type of device?
> > For example, this might make sense if a given device is from a
> > different, less trusted vendor.
>
> Is swiotlb commonly used for multiple devices that may be on different trust
> boundaries (and not behind a hardware iommu)?

Even a hardware iommu does not imply a 100% security from malicious
hardware. First lots of people use iommu=pt for performance reasons.
Second even without pt, unmaps are often batched, and sub-page buffers
might be used for DMA, so we are not 100% protected at all times.

> If so, then yes it sounds like a
> good application of multiple swiotlb pools.
>
> > All this can then maybe be hidden behind the DMA API.
>
> Won't we still need some changes to virtio to make use of its own pool (to
> bounce buffers)? Something similar to its own DMA ops proposed in this patch?

If you are doing this for all devices, you need to either find a way
to do this without chaning DMA ops, or by doing some automatic change
to all drivers.

> > > +void virtio_bounce_set_dma_ops(struct virtio_device *vdev)
> > > +{
> > > + if (!bounce_buf_paddr)
> > > + return;
> > > +
> > > + set_dma_ops(vdev->dev.parent, &virtio_dma_ops);
> >
> >
> > I don't think DMA API maintainers will be happy with new users
> > of set_dma_ops.
>
> Is there an alternate API that is more preffered?

all this is supposed to be part of DMA API itself. new drivers aren't
supposed to have custom DMA ops.

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