[PATCH 0/2] Fix incorrect warning from dma-debug

[Robin Murphy]

On 25/01/17 16:23, Geert Uytterhoeven wrote:
> Hi Robin,

Hi Geert,

> On Mon, May 9, 2016 at 11:37 AM, Robin Murphy <robin.murphy at arm.com> wrote:
>> On 08/05/16 11:59, Niklas Söderlund wrote:
>>> While using CONFIG_DMA_API_DEBUG i came across this warning which I
>>> think is a false positive. As shown dma_sync_single_for_device() are
>>> called from the dma_map_single() call path. This triggers the warning
>>> since the dma-debug code have not yet been made aware of the mapping.
>>
>> Almost right ;) The thing being mapped (the SPI device's buffer) and the
>> thing being synced (the IOMMU's PTE) are entirely unrelated. Due to the
>> current of_iommu_init() setup, the IOMMU is probed long before
>> dma_debug_init() gets called, therefore DMA debug is missing entries for
>> some of the initial page table mappings and gets confused when we update
>> them later.
> 
> I think I've been seeing the same as Niklas since quite a while.
> Finally I had a deeper look, and it looks like there is a bug somewhere,
> causing the wrong IOMMU PTE to be synced.
> 
>>> I try to solve this by introducing __dma_sync_single_for_device() which
>>> do not call into the dma-debug code. I'm no expert and this might be a
>>> bad way of solving the problem but it allowed me to keep working.
>>
>> The simple fix should be to just call dma_debug_init() from a sufficiently
>> earlier initcall level. The best would be to sort out a proper device
>> dependency order to avoid the whole early-IOMMU-creation thing entirely.
> 
> And so I did. After disabling the call to dma_debug_fs_init(), you can call
> dma_debug_init() quite early. But the warning didn't go away:

Yet the underlying reason has subtly changed!

>     ipmmu-vmsa e67b0000.mmu: DMA-API: device driver tries to sync DMA memory
>         it has not allocated [device address=0x000000067bab2ff8] [size=8 bytes]
>     ------------[ cut here ]------------
>     WARNING: CPU: 0 PID: 174 at lib/dma-debug.c:1235 check_sync+0xcc/0x568
>     ...
>     [<ffffff800823a3a4>] check_sync+0xcc/0x568
>     [<ffffff800823a8d0>] debug_dma_sync_single_for_device+0x44/0x4c
>     [<ffffff80082b8d34>] __arm_lpae_set_pte.isra.3+0x6c/0x78
>     [<ffffff80082b977c>] __arm_lpae_map+0x318/0x384
>     [<ffffff80082b9c58>] arm_lpae_map+0xb0/0xc4
>     [<ffffff80082bbc58>] ipmmu_map+0x48/0x58
>     [<ffffff80082b6754>] iommu_map+0x120/0x1fc
>     [<ffffff80082b7bc8>] __iommu_dma_map+0xb8/0xec
>     [<ffffff80082b8514>] iommu_dma_map_page+0x50/0x58
>     [<ffffff8008092d28>] __iommu_map_page+0x54/0x98
> 
> So, who allocated that memory?
> 
> During early kernel init (before fs_initcall(dma_debug_init)):
> 
>     arm-lpae io-pgtable: arm_lpae_alloc_pgtable:652: cfg->ias = 32

Note that you have a 32-bit IAS...

>         data->pg_shift = 12 va_bits = 20
>     arm-lpae io-pgtable: arm_lpae_alloc_pgtable:657: data->bits_per_level = 9
>         data->levels = 3 pgd_bits = 2
>     ipmmu-vmsa e67b0000.mmu: __arm_lpae_alloc_pages:224
>         dma_map_single(0xffffffc63bab2000, 32) returned 0x000000067bab2000
> 
> Hence 0x67bab2000 is the PGD, which has only 4 entries (32 bytes).
> Call stack:
> 
>     [<ffffff80082b9240>] __arm_lpae_alloc_pages.isra.11+0x144/0x1e8
>     [<ffffff80082b93c0>] arm_64_lpae_alloc_pgtable_s1+0xdc/0x118
>     [<ffffff80082b9440>] arm_32_lpae_alloc_pgtable_s1+0x44/0x68
>     [<ffffff80082b8b1c>] alloc_io_pgtable_ops+0x4c/0x80
>     [<ffffff80082bbf28>] ipmmu_attach_device+0xd0/0x3b0
> 
> When starting DMA from the device:
> 
>     iommu: map: iova 0xfffffff000 pa 0x000000067a555000 size 0x1000 pgsize 4096

...then count the f's carefully.

>     arm-lpae io-pgtable: __arm_lpae_map:318: iova 0xfffffff000
>         phys 0x000000067a555000 size 4096 lvl 1 ptep 0xffffffc63bab2000
>     arm-lpae io-pgtable: __arm_lpae_map:320: incr. ptep to 0xffffffc63bab2ff8
>     ipmmu-vmsa e67b0000.mmu: __arm_lpae_alloc_pages:224
>         dma_map_single(0xffffffc63a490000, 4096) returned 0x000000067a490000
>     ipmmu-vmsa e67b0000.mmu: DMA-API: device driver tries to sync DMA memory
>         it has not allocated [device address=0x000000067bab2ff8] [size=8 bytes]
> 
> __arm_lpae_map() added "ARM_LPAE_LVL_IDX(iova, lvl, data)" == 0xff8 to ptep
> (the PGD base address), but the PGD has only 32 bytes, leading to the warning.
> 
> Does my analysis make sense?
> Do you have a clue?

The initial false positive misleads from the fact that this is actually
DMA-debug doing its job admirably. The bug lies in however you ended up
trying to map a 40-bit IOVA in a 32-bit pagetable.

Robin.

> 
> Thanks!
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
>