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

[Robin Murphy]

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?

Robin.

> 
> Thanks,
> Jean-Philippe
> 
>> Cc: Andy Lutomirski <luto at kernel.org>
>> Cc: Michael S. Tsirkin <mst at redhat.com>
>> Signed-off-by: Will Deacon <will.deacon at arm.com>
>> ---
>>  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 ed9c9eeedfe5..06b91e29d1b7 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;
>>  }
>>  
>>
> 
> 
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