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

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Date:   Mon, 30 Jul 2018 02:30:27 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        aik@...abs.ru, robh@...nel.org, joe@...ches.com,
        elfring@...rs.sourceforge.net, david@...son.dropbear.id.au,
        jasowang@...hat.com, benh@...nel.crashing.org, mpe@...erman.id.au,
        hch@...radead.org, linuxram@...ibm.com, haren@...ux.vnet.ibm.com,
        paulus@...ba.org, srikar@...ux.vnet.ibm.com
Subject: Re: [RFC 2/4] virtio: Override device's DMA OPS with
 virtio_direct_dma_ops selectively

> > +
> > +	if (xen_domain())
> > +		goto skip_override;
> > +
> > +	if (virtio_has_iommu_quirk(dev))
> > +		set_dma_ops(dev->dev.parent, &virtio_direct_dma_ops);
> > +
> > + skip_override:
> > +
> 
> I prefer normal if scoping as opposed to goto spaghetti pls.
> Better yet move vring_use_dma_api here and use it.
> Less of a chance something will break.

I agree about avoid pointless gotos here, but we can do things
perfectly well without either gotos or a confusing helper here
if we structure it right. E.g.:

	// suitably detailed comment here
	if (!xen_domain() &&
	    !virtio_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM))
		set_dma_ops(dev->dev.parent, &virtio_direct_dma_ops);

and while we're at it - modifying dma ops for the parent looks very
dangerous.  I don't think we can do that, as it could break iommu
setup interactions.  IFF we set a specific dma map ops it has to be
on the virtio device itself, of which we have full control.