Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices

On Wed, 2018-05-23 at 21:50 +0300, Michael S. Tsirkin wrote:

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?

I thought I had replied to this...

There are a couple of reasons:

- First qemu doesn't know that the guest will switch to "secure mode"
in advance. There is no difference between a normal and a secure
partition until the partition does the magic UV call to "enter secure
mode" and qemu doesn't see any of it. So who can set the flag here ?

- Second, when using VIRTIO_F_IOMMU_PLATFORM, we also make qemu (or
vhost) go through the emulated MMIO for every access to the guest,
which adds additional overhead.

Cheers,
Ben.

quoted

 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.

quoted

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