Revert "vring: Force use of DMA API for ARM-based systems with legacy devices"

This reverts commit c707061.

This has been shown to regress on some ARM systems:

by forcing on DMA API usage for ARM systems, we have inadvertently
kicked open a hornets' nest in terms of cache-coherency. Namely that
unless the virtio device is explicitly described as capable of coherent
DMA by firmware, the DMA APIs on ARM and other DT-based platforms will
assume it is non-coherent. This turns out to cause a big problem for the
likes of QEMU and kvmtool, which generate virtio-mmio devices in their
guest DTs but neglect to add the often-overlooked "dma-coherent"
property; as a result, we end up with the guest making non-cacheable
accesses to the vring, the host doing so cacheably, both talking past
each other and things going horribly wrong.

We are working on a safer work-around.

Fixes: c707061 ("vring: Force use of DMA API for ARM-based systems with legacy devices")
Reported-by: Robin Murphy <[email protected]>
Cc: <[email protected]>
Signed-off-by: Will Deacon <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Acked-by: Marc Zyngier <[email protected]>

Author: mstsirkin

drivers/virtio/virtio_ring.c

@@ -159,13 +159,6 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
 	if (xen_domain())
 		return true;
 
-	/*
-	 * On ARM-based machines, the DMA ops will do the right thing,
-	 * so always use them with legacy devices.
-	 */
-	if (IS_ENABLED(CONFIG_ARM) || IS_ENABLED(CONFIG_ARM64))
-		return !virtio_has_feature(vdev, VIRTIO_F_VERSION_1);
-
 	return false;
 }