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

PATCH [email protected] 1 participant 1 message Jan 30, 2017

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2017-01-30 09:08 UTC #0 lore raw

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

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:
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.

Quoted 1 line

From c7070619f3408d9a0dffbed9149e6f00479cf43b Mon Sep 17 00:00:00 2001

From: Will Deacon <[email protected]>
Date: Fri, 20 Jan 2017 10:33:32 +0000
Subject: vring: Force use of DMA API for ARM-based systems with legacy devices

From: Will Deacon <[email protected]>

commit c7070619f3408d9a0dffbed9149e6f00479cf43b upstream.

Booting Linux on an ARM fastmodel containing an SMMU emulation results in an unexpected I/O page fault from the legacy virtio-blk PCI device:

[    1.211721] arm-smmu-v3 2b400000.smmu: event 0x10 received:
[    1.211800] arm-smmu-v3 2b400000.smmu:	0x00000000fffff010
[    1.211880] arm-smmu-v3 2b400000.smmu:	0x0000020800000000
[    1.211959] arm-smmu-v3 2b400000.smmu:	0x00000008fa081002
[    1.212075] arm-smmu-v3 2b400000.smmu:	0x0000000000000000
[    1.212155] arm-smmu-v3 2b400000.smmu: event 0x10 received:
[    1.212234] arm-smmu-v3 2b400000.smmu:	0x00000000fffff010
[    1.212314] arm-smmu-v3 2b400000.smmu:	0x0000020800000000
[    1.212394] arm-smmu-v3 2b400000.smmu:	0x00000008fa081000
[    1.212471] arm-smmu-v3 2b400000.smmu:	0x0000000000000000

This is because the legacy virtio-blk device is behind an SMMU, so we have consequently swizzled its DMA ops and configured the SMMU to translate accesses. This then requires the vring code to use the DMA API to establish translations, otherwise all transactions will result in fatal faults and termination.

Given that ARM-based systems only see an SMMU if one is really present (the topology is all described by firmware tables such as device-tree or IORT), then we can safely use the DMA API for all legacy virtio devices. Modern devices can advertise the prescense of an IOMMU using the VIRTIO_F_IOMMU_PLATFORM feature flag.

Cc Andy Lutomirski <[email protected]>

Cc Michael S. Tsirkin <[email protected]>

Fixes 876945dbf649 ("arm64: Hook up IOMMU dma_ops")

Signed-off-by Will Deacon <[email protected]>

Signed-off-by Michael S. Tsirkin <[email protected]>

Acked-by Marc Zyngier <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

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

drivers/virtio/virtio_ring.c +7 -0

@@ -159,6 +159,13 @@ 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/vring-force-use-of-dma-api-for-arm-based-systems-with-legacy-devices.patch