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Date:   Mon, 30 Jul 2018 02:25:51 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc:     virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
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        jasowang@redhat.com, benh@kernel.crashing.org, mpe@ellerman.id.au,
        mst@redhat.com, hch@infradead.org, linuxram@us.ibm.com,
        haren@linux.vnet.ibm.com, paulus@samba.org,
        srikar@linux.vnet.ibm.com
Subject: Re: [RFC 2/4] virtio: Override device's DMA OPS with
 virtio_direct_dma_ops selectively
Message-ID: <20180730092551.GB26245@infradead.org>
References: <20180720035941.6844-1-khandual@linux.vnet.ibm.com>
 <20180720035941.6844-3-khandual@linux.vnet.ibm.com>
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> +const struct dma_map_ops virtio_direct_dma_ops;

This belongs into a header if it is non-static.  If you only
use it in this file anyway please mark it static and avoid a forward
declaration.

> +
>  int virtio_finalize_features(struct virtio_device *dev)
>  {
>  	int ret = dev->config->finalize_features(dev);
> @@ -174,6 +176,9 @@ int virtio_finalize_features(struct virtio_device *dev)
>  	if (ret)
>  		return ret;
>  
> +	if (virtio_has_iommu_quirk(dev))
> +		set_dma_ops(dev->dev.parent, &virtio_direct_dma_ops);

This needs a big fat comment explaining what is going on here.

Also not new, but I find the existance of virtio_has_iommu_quirk and its
name horribly confusing.  It might be better to open code it here once
only a single caller is left.