linux-kernel - [RFC 3/4] virtio: Force virtio core to use DMA API callbacks for all virtio devices

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Date:   Fri, 20 Jul 2018 09:29:40 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Cc:     linuxppc-dev@...ts.ozlabs.org, aik@...abs.ru, robh@...nel.org,
        joe@...ches.com, elfring@...rs.sourceforge.net,
        david@...son.dropbear.id.au, jasowang@...hat.com,
        benh@...nel.crashing.org, mpe@...erman.id.au, mst@...hat.com,
        hch@...radead.org, khandual@...ux.vnet.ibm.com,
        linuxram@...ibm.com, haren@...ux.vnet.ibm.com, paulus@...ba.org,
        srikar@...ux.vnet.ibm.com
Subject: [RFC 3/4] virtio: Force virtio core to use DMA API callbacks for all virtio devices

Virtio core should use DMA API callbacks for all virtio devices which may
generate either GAP or IOVA depending on VIRTIO_F_IOMMU_PLATFORM flag and
resulting QEMU expectations. This implies that every virtio device needs
to have a DMA OPS structure. This removes previous GPA fallback code paths.

Signed-off-by: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
---
 drivers/virtio/virtio_ring.c | 65 ++------------------------------------------
 1 file changed, 2 insertions(+), 63 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 814b395..c265964 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -141,26 +141,6 @@ struct vring_virtqueue {
  * unconditionally on data path.
  */
 
-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;
-}
-
 /*
  * The DMA ops on various arches are rather gnarly right now, and
  * making all of the arch DMA ops work on the vring device itself
@@ -176,9 +156,6 @@ static dma_addr_t vring_map_one_sg(const struct vring_virtqueue *vq,
 				   struct scatterlist *sg,
 				   enum dma_data_direction direction)
 {
-	if (!vring_use_dma_api(vq->vq.vdev))
-		return (dma_addr_t)sg_phys(sg);
-
 	/*
 	 * We can't use dma_map_sg, because we don't use scatterlists in
 	 * the way it expects (we don't guarantee that the scatterlist
@@ -193,9 +170,6 @@ static dma_addr_t vring_map_single(const struct vring_virtqueue *vq,
 				   void *cpu_addr, size_t size,
 				   enum dma_data_direction direction)
 {
-	if (!vring_use_dma_api(vq->vq.vdev))
-		return (dma_addr_t)virt_to_phys(cpu_addr);
-
 	return dma_map_single(vring_dma_dev(vq),
 			      cpu_addr, size, direction);
 }
@@ -205,9 +179,6 @@ static void vring_unmap_one(const struct vring_virtqueue *vq,
 {
 	u16 flags;
 
-	if (!vring_use_dma_api(vq->vq.vdev))
-		return;
-
 	flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
 
 	if (flags & VRING_DESC_F_INDIRECT) {
@@ -228,9 +199,6 @@ static void vring_unmap_one(const struct vring_virtqueue *vq,
 static int vring_mapping_error(const struct vring_virtqueue *vq,
 			       dma_addr_t addr)
 {
-	if (!vring_use_dma_api(vq->vq.vdev))
-		return 0;
-
 	return dma_mapping_error(vring_dma_dev(vq), addr);
 }
 
@@ -1016,43 +984,14 @@ EXPORT_SYMBOL_GPL(__vring_new_virtqueue);
 static void *vring_alloc_queue(struct virtio_device *vdev, size_t size,
 			      dma_addr_t *dma_handle, gfp_t flag)
 {
-	if (vring_use_dma_api(vdev)) {
-		return dma_alloc_coherent(vdev->dev.parent, size,
+	return dma_alloc_coherent(vdev->dev.parent, size,
 					  dma_handle, flag);
-	} else {
-		void *queue = alloc_pages_exact(PAGE_ALIGN(size), flag);
-		if (queue) {
-			phys_addr_t phys_addr = virt_to_phys(queue);
-			*dma_handle = (dma_addr_t)phys_addr;
-
-			/*
-			 * Sanity check: make sure we dind't truncate
-			 * the address.  The only arches I can find that
-			 * have 64-bit phys_addr_t but 32-bit dma_addr_t
-			 * are certain non-highmem MIPS and x86
-			 * configurations, but these configurations
-			 * should never allocate physical pages above 32
-			 * bits, so this is fine.  Just in case, throw a
-			 * warning and abort if we end up with an
-			 * unrepresentable address.
-			 */
-			if (WARN_ON_ONCE(*dma_handle != phys_addr)) {
-				free_pages_exact(queue, PAGE_ALIGN(size));
-				return NULL;
-			}
-		}
-		return queue;
-	}
 }
 
 static void vring_free_queue(struct virtio_device *vdev, size_t size,
 			     void *queue, dma_addr_t dma_handle)
 {
-	if (vring_use_dma_api(vdev)) {
-		dma_free_coherent(vdev->dev.parent, size, queue, dma_handle);
-	} else {
-		free_pages_exact(queue, PAGE_ALIGN(size));
-	}
+	dma_free_coherent(vdev->dev.parent, size, queue, dma_handle);
 }
 
 struct virtqueue *vring_create_virtqueue(
-- 
2.9.3