On 07/23/2018 07:46 AM, Anshuman Khandual wrote:

On 07/20/2018 06:45 PM, Michael S. Tsirkin wrote:

quoted

On Fri, Jul 20, 2018 at 09:29:41AM +0530, Anshuman Khandual wrote:

quoted

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

s/virito/virtio/

Oops, will fix it. Thanks for pointing out.

quoted

quoted

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 <redacted>
---
 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?

Not other than this one. But in the proposed implementation of
platform_override_dma_ops on powerpc, we will again check on
virtio_has_iommu_quirk before overriding it with SWIOTLB.

void platform_override_dma_ops(struct virtio_device *vdev)
{
        if (is_ultravisor_platform() && virtio_has_iommu_quirk(vdev))
                set_dma_ops(vdev->dev.parent, &swiotlb_dma_ops);
}

quoted

If so, we could put this into virtio_has_iommu_quirk then.

Did you mean platform_override_dma_ops instead ? If so, yes that
is possible. Default implementation of platform_override_dma_ops
should just check on VIRTIO_F_IOMMU_PLATFORM feature and override
with virtio_direct_dma_ops but arch implementation can check on
what ever else they would like and override appropriately.

Default platform_override_dma_ops will be like this

#ifndef platform_override_dma_ops
static inline void platform_override_dma_ops(struct virtio_device *vdev)
{
	if(!virtio_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM))
		set_dma_ops(dev->dev.parent, &virtio_direct_dma_ops);
}
#endif

Proposed powerpc implementation will be like this instead

void platform_override_dma_ops(struct virtio_device *vdev)
{
	if (virtio_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM))
		return;

        if (is_ultravisor_platform())
                set_dma_ops(vdev->dev.parent, &swiotlb_dma_ops);
	else
		set_dma_ops(dev->dev.parent, &virtio_direct_dma_ops);
	
}

There is a redundant definition of virtio_has_iommu_quirk in the tools
directory (tools/virtio/linux/virtio_config.h) which does not seem to
be used any where. I guess that can be removed without problem.

Does this sound okay ? It will merge patch 3 and 4 into a single one.
On the other hand it also passes the responsibility of dealing with
VIRTIO_F_IOMMU_PLATFORM flag to the architecture callback which might
be bit problematic. Keeping VIRTIO_F_IOMMU_PLATFORM handling in virtio
core at least makes sure that the device has a working DMA ops to fall
back on if the arch hook fails to take care of it somehow.