Re: [PATCH 0/3] Fix virtio-blk issue with SWIOTLB

[Christoph Hellwig <hch@lst.de>]

On Mon, Jan 14, 2019 at 07:12:08PM +0000, Robin Murphy wrote:
> Now, in theory, yes, the real issue at hand is not unique to virtio-blk nor 
> SEV - any driver whose device has a sufficiently large DMA segment size and 
> who manages to get sufficient physically-contiguous memory could 
> technically generate a scatterlist segment longer than SWIOTLB can handle. 
> However, in practice that basically never happens, not least because very 
> few drivers ever override the default 64K DMA segment limit. AFAICS nothing 
> in drivers/virtio is calling dma_set_max_seg_size() or otherwise assigning 
> any dma_parms to replace the defaults either, so the really interesting 
> question here is how are these apparently-out-of-spec 256K segments getting 
> generated at all?

drivers/block/virtio_blk.c:virtblk_probe():

	/* Host can optionally specify maximum segment size and number of
         * segments. */
	err = virtio_cread_feature(vdev, VIRTIO_BLK_F_SIZE_MAX,
				   struct virtio_blk_config, size_max, &v);
        if (!err)
		blk_queue_max_segment_size(q, v);
	else
		blk_queue_max_segment_size(q, -1U);

So it really is virtio_blk that is special here.  The host could
set VIRTIO_BLK_F_SIZE_MAX to paper over the problem, but I really
think we need a dma_max_segment_size API that is wired up through
the dma_map_ops to sort this out for real.