On 24/01/17 16:06, Michael S. Tsirkin wrote:

On Tue, Jan 24, 2017 at 04:04:11PM +0000, Marc Zyngier wrote:

quoted

On 20/01/17 10:33, Will Deacon wrote:

quoted

From 213bad7fdb8e4f45a7724be169cda292bbb50d2b Mon Sep 17 00:00:00 2001
From: Will Deacon <redacted>
Date: Tue, 20 Dec 2016 12:12:49 +0000
Subject: [PATCH] vring: Force use of DMA API for ARM-based systems with legacy
 devices

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

<system hangs failing to read partition table>

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 <luto@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: <redacted>
Fixes: 876945dbf649 ("arm64: Hook up IOMMU dma_ops")
Signed-off-by: Will Deacon <redacted>
---
 drivers/virtio/virtio_ring.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 409aeaa49246..7e38ed79c3fc 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -159,6 +159,13 @@ 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;
 }
 

Acked-by: Marc Zyngier <redacted>

Any chance this fix (or anything with similar effects) gets applied
sometime soon? I cannot use the model without using a similar
workaround:

http://git.kernel.org/cgit/linux/kernel/git/maz/arm-platforms.git/commit/?h=kvm-arm64/gicv4-wip&id=622ff1190890c0ae60d57e76a7c2f3e6fb27e25d

and I suspect that other users of the same system are carrying their own
version of the fix. Something in mainline would be infinitely better.

Thanks,

	M.

I'll merge this in the next pull.

Awesome, thanks a lot.

	M.
-- 
Jazz is not dead. It just smells funny...