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

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Date:   Wed, 29 Apr 2020 09:39:29 +0530
From:   Srivatsa Vaddagiri <vatsa@...eaurora.org>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>, konrad.wilk@...cle.com,
        jasowang@...hat.com, jan.kiszka@...mens.com, will@...nel.org,
        stefano.stabellini@...inx.com, iommu@...ts.linux-foundation.org,
        virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org, tsoni@...eaurora.org,
        pratikp@...eaurora.org, christoffer.dall@....com,
        alex.bennee@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] virtio: Add bounce DMA ops

* Stefano Stabellini <sstabellini@...nel.org> [2020-04-28 16:04:34]:

> > > Is swiotlb commonly used for multiple devices that may be on different trust
> > > boundaries (and not behind a hardware iommu)?
> 
> The trust boundary is not a good way of describing the scenario and I
> think it leads to miscommunication.
> 
> A better way to describe the scenario would be that the device can only
> DMA to/from a small reserved-memory region advertised on device tree.
> 
> Do we have other instances of devices that can only DMA to/from very
> specific and non-configurable address ranges? If so, this series could
> follow their example.

AFAICT there is no such notion in current DMA API.

static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size,
                bool is_ram)
{
        return end <= min_not_zero(*dev->dma_mask, dev->bus_dma_limit);
}

Only the max address a device can access is defined and not a range that we seem
to need here. I think we need to set the bus_dma_limit to 0 for virtio devices
which will force the use of swiotlb_map API. We should also have a per-device
swiotlb pool defined, so that swiotlb can use the pool meant for the given
device.

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