Patch "Revert "vring: Force use of DMA API for ARM-based systems with legacy devices"" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
revert-vring-force-use-of-dma-api-for-arm-based-systems-with-legacy-devices.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Michael S. Tsirkin" <[email protected]>
Date: Fri, 3 Feb 2017 05:43:52 +0200
Subject: Revert "vring: Force use of DMA API for ARM-based systems with legacy devices"

From: Michael S. Tsirkin <[email protected]>

commit 0d5415b489f68b58e1983a53793d25d53098ed4b upstream.

This reverts commit c7070619f3408d9a0dffbed9149e6f00479cf43b.

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.

---
 drivers/virtio/virtio_ring.c |    7 -------
 1 file changed, 7 deletions(-)

@@ -159,13 +159,6 @@ static bool vring_use_dma_api(struct vir
 	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;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/vhost-fix-initialization-for-vq-is_le.patch queue-4.9/revert-vring-force-use-of-dma-api-for-arm-based-systems-with-legacy-devices.patch