linux-kernel - Re: [RFC 2/4] virtio: Override device's DMA OPS with virtio_direct_dma

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Message-Id: <1e197586-4e11-48e1-a0dd-c2c440c64f9e@linux.vnet.ibm.com>
Date:   Tue, 31 Jul 2018 12:30:19 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     robh@...nel.org, srikar@...ux.vnet.ibm.com, mst@...hat.com,
        aik@...abs.ru, jasowang@...hat.com, linuxram@...ibm.com,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, paulus@...ba.org,
        joe@...ches.com, linuxppc-dev@...ts.ozlabs.org,
        elfring@...rs.sourceforge.net, haren@...ux.vnet.ibm.com,
        david@...son.dropbear.id.au
Subject: Re: [RFC 2/4] virtio: Override device's DMA OPS with
 virtio_direct_dma_ops selectively

On 07/30/2018 02:55 PM, Christoph Hellwig wrote:
>> +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.

Sure, will make it static, move the definition up in the file to avoid
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.

Sure, will do. Also talk about the XEN domain exception as well once
that goes into this conditional statement.

> 
> 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.

Sure will do. There is one definition in the tools directory which can
be removed and then this will be the only one left.