Re: [PATCH v2 2/2] vring: Force use of DMA API for ARM-based systems
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2017-01-10 23:33:31
Also in:
linux-arm-kernel
On Tue, Jan 10, 2017 at 05:51:18PM +0000, Robin Murphy wrote:
From: Will Deacon <redacted> 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 virtio-blk 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 virtio devices. Cc: Andy Lutomirski <luto@kernel.org> Cc: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Will Deacon <redacted>
I'd like to better understand then need for this one. Can't the device in question just set VIRTIO_F_IOMMU_PLATFORM ? I'd rather we avoided need for more hacks and just have everyone switch to that.
quoted hunk ↗ jump to hunk
--- drivers/virtio/virtio_ring.c | 4 ++++ 1 file changed, 4 insertions(+)diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index 409aeaa49246..447245f2c813 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c@@ -159,6 +159,10 @@ 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 */ + if (IS_ENABLED(CONFIG_ARM) || IS_ENABLED(CONFIG_ARM64)) + return true; + return false; }-- 2.10.2.dirty