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SubjectRe: [RFC V2] virtio: Add platform specific DMA API translation for virito devices
subj: s/virito/virtio/

On Tue, May 22, 2018 at 12:03:17PM +0530, Anshuman Khandual wrote:
> This adds a hook which a platform can define in order to allow it to
> force the use of the DMA API for all virtio devices even if they don't
> have the VIRTIO_F_IOMMU_PLATFORM flag set. 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 <khandual@linux.vnet.ibm.com>
> ---
> Changes in V2:
>
> The arch callback has been enabled through an weak symbol defintion
> so that it is enabled only for those architectures subscribing to
> this new framework. Clarified the patch description. The primary
> objective for this RFC has been to get an in principle agreement
> on this approach.
>
> Original V1:
>
> Original RFC and discussions https://patchwork.kernel.org/patch/10324405/

I re-read that discussion and I'm still unclear on the
original question, since I got several apparently
conflicting answers.

I asked:

Why isn't setting VIRTIO_F_IOMMU_PLATFORM on the
hypervisor side sufficient?



> arch/powerpc/include/asm/dma-mapping.h | 6 ++++++
> arch/powerpc/platforms/pseries/iommu.c | 11 +++++++++++
> drivers/virtio/virtio_ring.c | 10 ++++++++++
> 3 files changed, 27 insertions(+)
>
> diff --git a/arch/powerpc/include/asm/dma-mapping.h b/arch/powerpc/include/asm/dma-mapping.h
> index 8fa3945..056e578 100644
> --- a/arch/powerpc/include/asm/dma-mapping.h
> +++ b/arch/powerpc/include/asm/dma-mapping.h
> @@ -115,4 +115,10 @@ extern u64 __dma_get_required_mask(struct device *dev);
> #define ARCH_HAS_DMA_MMAP_COHERENT
>
> #endif /* __KERNEL__ */
> +
> +#define platform_forces_virtio_dma platform_forces_virtio_dma
> +
> +struct virtio_device;
> +
> +extern bool platform_forces_virtio_dma(struct virtio_device *vdev);
> #endif /* _ASM_DMA_MAPPING_H */
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index 06f0296..a2ec15a 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 +1397,13 @@ static int __init disable_multitce(char *str)
> __setup("multitce=", disable_multitce);
>
> machine_subsys_initcall_sync(pseries, tce_iommu_bus_notifier_init);
> +
> +bool platform_forces_virtio_dma(struct virtio_device *vdev)
> +{
> + /*
> + * On protected guest platforms, force virtio core to use DMA
> + * MAP API for all virtio devices. But there can also be some
> + * exceptions for individual devices like virtio balloon.
> + */
> + return (of_find_compatible_node(NULL, NULL, "ibm,ultravisor") != NULL);
> +}

Isn't this kind of slow? vring_use_dma_api is on
data path and supposed to be very fast.

> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 21d464a..47ea6c3 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -141,8 +141,18 @@ struct vring_virtqueue {
> * unconditionally on data path.
> */
>
> +#ifndef platform_forces_virtio_dma
> +static inline bool platform_forces_virtio_dma(struct virtio_device *vdev)
> +{
> + return false;
> +}
> +#endif
> +
> static bool vring_use_dma_api(struct virtio_device *vdev)
> {
> + if (platform_forces_virtio_dma(vdev))
> + return true;
> +
> if (!virtio_has_iommu_quirk(vdev))
> return true;
>
> --
> 2.9.3

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