[RFC PATCH] vring: Force use of DMA API for ARM-based systems

[Robin Murphy]

On 06/01/17 21:51, Andy Lutomirski wrote:
> On Fri, Jan 6, 2017 at 10:32 AM, Robin Murphy <robin.murphy at arm.com> wrote:
>> On 06/01/17 17:48, Jean-Philippe Brucker wrote:
>>> Hi Will,
>>>
>>> On 20/12/16 15:14, Will Deacon wrote:
>>>> 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.
>>>
>>> There is a problem with the platform block device on that same model.
>>> Since it's not behind the SMMU, the DMA ops fall back to swiotlb, which
>>> limits the number of mappings.
>>>
>>> It used to work with 4.9, but since 9491ae4 ("mm: don't cap request size
>>> based on read-ahead setting") unlocked read-ahead, we quickly run into
>>> the limit of swiotlb and panic:
>>>
>>> [    5.382359] virtio-mmio 1c130000.virtio_block: swiotlb buffer is full
>>> (sz: 491520 bytes)
>>> [    5.382452] virtio-mmio 1c130000.virtio_block: DMA: Out of SW-IOMMU
>>> space for 491520 bytes
>>> [    5.382531] Kernel panic - not syncing: DMA: Random memory could be
>>> DMA written
>>> ...
>>> [    5.383148] [<ffff0000083ad754>] swiotlb_map_page+0x194/0x1a0
>>> [    5.383226] [<ffff000008096bb8>] __swiotlb_map_page+0x20/0x88
>>> [    5.383320] [<ffff0000084bf738>] vring_map_one_sg.isra.1+0x70/0x88
>>> [    5.383417] [<ffff0000084c04fc>] virtqueue_add_sgs+0x2ec/0x4e8
>>> [    5.383505] [<ffff00000856d99c>] __virtblk_add_req+0x9c/0x1a8
>>> ...
>>> [    5.384449] [<ffff0000081829c4>] ondemand_readahead+0xfc/0x2b8
>>>
>>> Commit 9491ae4 caps the read-ahead request to a limit set by the backing
>>> device. For virtio-blk, it is infinite (as set by the call to
>>> blk_queue_max_hw_sectors in virtblk_probe).
>>>
>>> I'm not sure how to fix this. Setting an arbitrary sector limit in the
>>> virtio-blk driver seems unfair to other users. Maybe we should check if
>>> the device is behind a hardware IOMMU before using the DMA API?
>>
>> Hmm, this looks more like the virtio_block device simply has the wrong
>> DMA mask to begin with. For virtio-pci we set the streaming DMA mask to
>> 64 bits - should a platform device not be similarly capable?
> 
> If it's not, then turning off DMA API will cause random corruption.
> ISTM one way or another the bug is in either the DMA ops or in the
> driver initialization.

OK, having looked a little deeper, I reckon virtio_mmio_probe() is
indeed missing a dma_set_mask() call compared to its PCI friends. The
only question then is where does virtio-mmio stand with respect to
legacy/modern/44-bit/64-bit etc.?

Robin.

> 
> --Andy
>