Commit 0d5415b4 authored by Michael S. Tsirkin's avatar Michael S. Tsirkin

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

This reverts commit c7070619.

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: c7070619 ("vring: Force use of DMA API for ARM-based systems with legacy devices")
Reported-by: Robin Murphy's avatarRobin Murphy <>
Cc: <>
Signed-off-by: default avatarWill Deacon <>
Signed-off-by: Michael S. Tsirkin's avatarMichael S. Tsirkin <>
Acked-by: default avatarMarc Zyngier <>
parent 566cf877
......@@ -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.
return !virtio_has_feature(vdev, VIRTIO_F_VERSION_1);
return false;
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