Subject: Re: [RFC 4/4] virtio: Add platform specific DMA API translation for virito devices

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 20 Jul 2018 16:15:28 +0300

On Fri, Jul 20, 2018 at 09:29:41AM +0530, Anshuman Khandual wrote:
>Subject: Re: [RFC 4/4] virtio: Add platform specific DMA API translation for
> virito devices

s/virito/virtio/

> This adds a hook which a platform can define in order to allow it to
> override virtio device's DMA OPS irrespective of whether it has the
> flag VIRTIO_F_IOMMU_PLATFORM set or not. We want to use this to do
> bounce-buffering of data on the new secure pSeries platform, currently
> under development, where a KVM host cannot access all of the memory
> space of a secure KVM guest.  The host can only access the pages which
> the guest has explicitly requested to be shared with the host, thus
> the virtio implementation in the guest has to copy data to and from
> shared pages.
> 
> With this hook, the platform code in the secure guest can force the
> use of swiotlb for virtio buffers, with a back-end for swiotlb which
> will use a pool of pre-allocated shared pages.  Thus all data being
> sent or received by virtio devices will be copied through pages which
> the host has access to.
> 
> Signed-off-by: Anshuman Khandual <[email-masked]>
> ---
>  arch/powerpc/include/asm/dma-mapping.h | 6 ++++++
>  arch/powerpc/platforms/pseries/iommu.c | 6 ++++++
>  drivers/virtio/virtio.c                | 7 +++++++
>  3 files changed, 19 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/dma-mapping.h b/arch/powerpc/include/asm/dma-mapping.h
> index 8fa3945..bc5a9d3 100644
> --- a/arch/powerpc/include/asm/dma-mapping.h
> +++ b/arch/powerpc/include/asm/dma-mapping.h
> @@ -116,3 +116,9 @@ extern u64 __dma_get_required_mask(struct device *dev);
>  
>  #endif /* __KERNEL__ */
>  #endif	/* _ASM_DMA_MAPPING_H */
> +
> +#define platform_override_dma_ops platform_override_dma_ops
> +
> +struct virtio_device;
> +
> +extern void platform_override_dma_ops(struct virtio_device *vdev);
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index 06f0296..5773bc7 100644
> --- a/arch/powerpc/platforms/pseries/iommu.c
> +++ b/arch/powerpc/platforms/pseries/iommu.c
> @@ -38,6 +38,7 @@
>  #include <linux/of.h>
>  #include <linux/iommu.h>
>  #include <linux/rculist.h>
> +#include <linux/virtio.h>
>  #include <asm/io.h>
>  #include <asm/prom.h>
>  #include <asm/rtas.h>
> @@ -1396,3 +1397,8 @@ static int __init disable_multitce(char *str)
>  __setup("multitce=", disable_multitce);
>  
>  machine_subsys_initcall_sync(pseries, tce_iommu_bus_notifier_init);
> +
> +void platform_override_dma_ops(struct virtio_device *vdev)
> +{
> +	/* Override vdev->parent.dma_ops if required */
> +}
> diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> index 6b13987..432c332 100644
> --- a/drivers/virtio/virtio.c
> +++ b/drivers/virtio/virtio.c
> @@ -168,6 +168,12 @@ EXPORT_SYMBOL_GPL(virtio_add_status);
>  
>  const struct dma_map_ops virtio_direct_dma_ops;
>  
> +#ifndef platform_override_dma_ops
> +static inline void platform_override_dma_ops(struct virtio_device *vdev)
> +{
> +}
> +#endif
> +
>  int virtio_finalize_features(struct virtio_device *dev)
>  {
>  	int ret = dev->config->finalize_features(dev);
> @@ -179,6 +185,7 @@ int virtio_finalize_features(struct virtio_device *dev)
>  	if (virtio_has_iommu_quirk(dev))
>  		set_dma_ops(dev->dev.parent, &virtio_direct_dma_ops);
>  
> +	platform_override_dma_ops(dev);

Is there a single place where virtio_has_iommu_quirk is called now?
If so, we could put this into virtio_has_iommu_quirk then.

>  	if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
>  		return 0;
>  
> -- 
> 2.9.3