Linux Virtualization - Apr 2023 - [PATCH vhost v6 08/11] virtio_ring: introduce virtqueue_dma_dev()

On Tue, Apr 11, 2023 at 11:56:47AM +0800, Jason Wang
wrote:
> > The DMA device for virtio_pci is the underlying PCI device, always.
> > !VIRTIO_F_ACCESS_PLATFORM means there is no dma device at all. 
Because
> > of all these things you can't just expose a pointer to the
dma_device
> > as that is just a completely wrong way of thinking about the problem.
> 
> Ok, so if there's no DMA at all we should avoid using the DMA API
> completely. This means we should check dma_dev against NULL in
> virtio_has_dma_quirk().
No nee to add a check to virtio_has_dma_quirk.

But looking at virtio_has_dma_quirk shows that virtio-gpu is
pretty messed up already as well.

It can't really poke into the DMA details.  We'll need core virtio
helpers for allocating and syncing a sg_table instead and make
virtio_has_dma_quirk private to the core.

On Tue, Apr 11, 2023 at 12:10?PM Christoph Hellwig <hch at infradead.org>
wrote:
>
> On Tue, Apr 11, 2023 at 11:56:47AM +0800, Jason Wang wrote:
> > > The DMA device for virtio_pci is the underlying PCI device,
always.
> > > !VIRTIO_F_ACCESS_PLATFORM means there is no dma device at all. 
Because
> > > of all these things you can't just expose a pointer to the
dma_device
> > > as that is just a completely wrong way of thinking about the
problem.
> >
> > Ok, so if there's no DMA at all we should avoid using the DMA API
> > completely. This means we should check dma_dev against NULL in
> > virtio_has_dma_quirk().
>
> No nee to add a check to virtio_has_dma_quirk.
Ok, just to clarify, I meant there could be a case where the virtqueue
is emulated by software, in this case we need check whether the
virtqueue has a dma device or not in vring_use_dma_api(). If not we
need return false.
>
> But looking at virtio_has_dma_quirk shows that virtio-gpu is
> pretty messed up already as well.
>
> It can't really poke into the DMA details.  We'll need core virtio
> helpers for allocating and syncing a sg_table instead and make
> virtio_has_dma_quirk private to the core.
Adding Gerd.

Thanks
>