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Date:   Tue, 28 Apr 2020 16:41:04 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Srivatsa Vaddagiri <vatsa@codeaurora.org>
Cc:     konrad.wilk@oracle.com, jasowang@redhat.com,
        jan.kiszka@siemens.com, will@kernel.org,
        stefano.stabellini@xilinx.com, iommu@lists.linux-foundation.org,
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        pratikp@codeaurora.org, christoffer.dall@arm.com,
        alex.bennee@linaro.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 5/5] virtio: Add bounce DMA ops
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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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