Subject: Re: [RFC 2/4] virtio: Override device's DMA OPS with virtio_direct_dma_ops selectively

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 29 Jul 2018 00:16:19 +0300

On Sat, Jul 28, 2018 at 02:26:24PM +0530, Anshuman Khandual wrote:
> On 07/20/2018 09:29 AM, Anshuman Khandual wrote:
> > Now that virtio core always needs all virtio devices to have DMA OPS, we
> > need to make sure that the structure it points is the right one. In the
> > absence of VIRTIO_F_IOMMU_PLATFORM flag QEMU expects GPA from guest kernel.
> > In such case, virtio device must use default virtio_direct_dma_ops DMA OPS
> > structure which transforms scatter gather buffer addresses as GPA. This
> > DMA OPS override must happen as early as possible during virtio device
> > initializatin sequence before virtio core starts using given device's DMA
> > OPS callbacks for I/O transactions. This change detects device's IOMMU flag
> > and does the override in case the flag is cleared.
> > 
> > Signed-off-by: Anshuman Khandual <[email-masked]>
> > ---
> >  drivers/virtio/virtio.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > index 7907ad3..6b13987 100644
> > --- a/drivers/virtio/virtio.c
> > +++ b/drivers/virtio/virtio.c
> > @@ -166,6 +166,8 @@ void virtio_add_status(struct virtio_device *dev, unsigned int status)
> >  }
> >  EXPORT_SYMBOL_GPL(virtio_add_status);
> > 
> > +const struct dma_map_ops virtio_direct_dma_ops;
> > +
> >  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;
> 
> 
> The previous patch removed the code block for XEN guests which forced
> the use of DMA API all the time irrespective of VIRTIO_F_IOMMU_PLATFORM
> flag on the device. Here is what I have removed with patch 2/4 which
> breaks the existing semantics on XEN guests.
> 
> -static bool vring_use_dma_api(struct virtio_device *vdev)
> -{
> -	if (!virtio_has_iommu_quirk(vdev))
> -		return true;
> -
> -	/* Otherwise, we are left to guess. */
> -	/*
> -	 * In theory, it's possible to have a buggy QEMU-supposed
> -	 * emulated Q35 IOMMU and Xen enabled at the same time.  On
> -	 * such a configuration, virtio has never worked and will
> -	 * not work without an even larger kludge.  Instead, enable
> -	 * the DMA API if we're a Xen guest, which at least allows
> -	 * all of the sensible Xen configurations to work correctly.
> -	 */
> -	if (xen_domain())
> -		return true;
> -
> -	return false;
> -}
> 
> XEN guests would not like override with virtio_direct_dma_ops in any
> case irrespective of the flag VIRTIO_F_IOMMU_PLATFORM. So the existing
> semantics can be preserved with something like this. It just assumes
> that dev->dma_ops is non-NULL and a valid one set by the architecture.
> If required we can add those tests here before skipping the override.
> 
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 7907ad3..6b13987 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -166,6 +166,8 @@ void virtio_add_status(struct virtio_device *dev, unsigned int status)
>  }
>  EXPORT_SYMBOL_GPL(virtio_add_status);
> 
> +const struct dma_map_ops virtio_direct_dma_ops;
> +
>  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 (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.

>  	if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
>  		return 0
> 
> Will incorporate these changes in the next version.