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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jan 09, 2017 at 11:24:04AM +0000, Robin Murphy wrote:
> On 06/01/17 21:51, Andy Lutomirski wrote:
> > On Fri, Jan 6, 2017 at 10:32 AM, Robin Murphy <robin.murphy@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.

AFAIK current drivers support the modern interface since Jan 2015.

44/64 is almost the same as PCI really, except page size isn't fixed to 4K.
So legacy ones need to set coherent mask to 32 + PAGE_SHIFT.


> > 
> > --Andy
> >