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To:     Robin Murphy <robin.murphy@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>
Cc:     Mike Rapoport <rppt@kernel.org>, Alex Bee <knaerzche@gmail.com>,
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)
Message-ID: <aff3942e-b9ce-5bae-8214-0e5d89cd071c@redhat.com>
Date:   Wed, 25 Aug 2021 13:21:16 +0200
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On 25.08.21 12:58, Robin Murphy wrote:
> On 2021-08-25 11:38, David Hildenbrand wrote:
>> On 25.08.21 12:20, Catalin Marinas wrote:
>>> + hch
>>>
>>> On Tue, Aug 24, 2021 at 08:59:22PM +0200, David Hildenbrand wrote:
>>>> On 24.08.21 20:46, Robin Murphy wrote:
>>>>> On 2021-08-24 19:28, Mike Rapoport wrote:
>>>>>> On Tue, Aug 24, 2021 at 06:37:41PM +0100, Catalin Marinas wrote:
>>>>>>> On Tue, Aug 24, 2021 at 03:40:47PM +0200, Alex Bee wrote:
>>>>>>>> it seems there is a regression in arm64 memory mapping in 5.14,
>>>>>>>> since it
>>>>>>>> fails on Rockchip RK3328 when the pl330 dmac tries to map with:
>>>>>>>>
>>>>>>>>    ------------[ cut here ]------------
>>>>>>>>    WARNING: CPU: 2 PID: 373 at kernel/dma/mapping.c:235
>>>>>>>> dma_map_resource+0x68/0xc0
>>>>>>>>    Modules linked in: spi_rockchip(+) fuse
>>>>>>>>    CPU: 2 PID: 373 Comm: systemd-udevd Not tainted 5.14.0-rc7 #1
>>>>>>>>    Hardware name: Pine64 Rock64 (DT)
>>>>>>>>    pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--)
>>>>>>>>    pc : dma_map_resource+0x68/0xc0
>>>>>>>>    lr : pl330_prep_slave_fifo+0x78/0xd0
>>>>>>>>    sp : ffff800012102ae0
>>>>>>>>    x29: ffff800012102ae0 x28: ffff000005c94800 x27: 0000000000000000
>>>>>>>>    x26: ffff000000566bd0 x25: 0000000000000001 x24: 0000000000000001
>>>>>>>>    x23: 0000000000000002 x22: ffff000000628c00 x21: 0000000000000001
>>>>>>>>    x20: ffff000000566bd0 x19: 0000000000000001 x18: 0000000000000000
>>>>>>>>    x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
>>>>>>>>    x14: 0000000000000277 x13: 0000000000000001 x12: 0000000000000000
>>>>>>>>    x11: 0000000000000001 x10: 00000000000008e0 x9 : ffff800012102a80
>>>>>>>>    x8 : ffff000000d14b80 x7 : ffff0000fe7b12f0 x6 : ffff0000fe7b1100
>>>>>>>>    x5 : fffffc000000000f x4 : 0000000000000000 x3 : 0000000000000001
>>>>>>>>    x2 : 0000000000000001 x1 : 00000000ff190800 x0 : ffff000000628c00
>>>>>>>>    Call trace:
>>>>>>>>      dma_map_resource+0x68/0xc0
>>>>>>>>      pl330_prep_slave_sg+0x58/0x220
>>>>>>>>      rockchip_spi_prepare_dma+0xd8/0x2c0 [spi_rockchip]
>>>>>>>>      rockchip_spi_transfer_one+0x294/0x3d8 [spi_rockchip]
>>>>>>> [...]
>>>>>>>> Note: This does not relate to the spi driver - when disabling
>>>>>>>> this device in
>>>>>>>> the device tree it fails for any other (i2s, for instance) which
>>>>>>>> uses dma.
>>>>>>>> Commenting out the failing check at [1], however, helps and the
>>>>>>>> mapping
>>>>>>>> works again.
>>>>>>
>>>>>>> Do you know which address dma_map_resource() is trying to map (maybe
>>>>>>> add some printk())? It's not supposed to map RAM, hence the warning.
>>>>>>> Random guess, the address is 0xff190800 (based on the x1 above but
>>>>>>> the
>>>>>>> regs might as well be mangled).
>>>>>>
>>>>>> 0xff190800 will cause this warning for sure. It has a memory map,
>>>>>> but it is
>>>>>> not RAM so old version of pfn_valid() would return 0 and the new one
>>>>>> returns 1.
>>>>>
>>>>> How does that happen, though? It's not a memory address, and it's not
>>>>> even within the bounds of anywhere there should or could be memory.
>>>>> This
>>>>> SoC has a simple memory map - everything from 0 to 0xfeffffff goes to
>>>>> the DRAM controller (which may not all be populated, and may have
>>>>> pieces
>>>>> carved out by secure firmware), while 0xff000000-0xffffffff is MMIO.
>>>>> Why
>>>>> do we have pages (or at least the assumption of pages) for somewhere
>>>>> which by all rights should not have them?
>>>>
>>>> Simple: we allocate the vmemmap for whole sections (e.g., 128 MiB) to
>>>> avoid
>>>> any such hacks. If there is a memory hole, it gets a memmap as well.
> 
> Urgh, apologies for being slow. This case isn't a memory hole as such,
> but I failed to consider the *ends* of memory not being section-aligned
> and leading to an overlap anyway.
> 
>>>> Tricking pfn_valid() into returning "false" where we actually have a
>>>> memmap
>>>> only makes it look like there is no memmap; but there is one, and
>>>> it's PG_reserved.
>>>
>>> I can see the documentation for pfn_valid() does not claim anything more
>>> than the presence of an memmap entry. But I wonder whether the confusion
>>> is wider-spread than just the DMA code. At a quick grep, try_ram_remap()
>>> assumes __va() can be used on pfn_valid(), though I suspect it relies on
>>> the calling function to check that the resource was RAM. The arm64
>>> kern_addr_valid() returns true based on pfn_valid() and kcore.c uses
>>> standard memcpy on it, which wouldn't work for I/O (should we change
>>> this check to pfn_is_map_memory() for arm64?).
>>>
>>
>> kern_addr_valid() checks that there is a direct map entry, and that the
>> mapped address has a valid mmap. (copied from x86-64)
>>
>> Would you expect to have a direct map for memory holes and similar (IOW,
>> !System RAM)?
> 
> Probably - we can have no-map regions for firmware-reserved RAM which I
> believe end up as PG_reserved if they're small enough, for the same
> reasoning as this case.
> 
>>>>>>> Either pfn_valid() gets confused in 5.14 or something is wrong
>>>>>>> with the
>>>>>>> DT. I have a suspicion it's the former since reverting the above
>>>>>>> commit
>>>>>>> makes it disappear.
>>>>>>
>>>>>> I think pfn_valid() actually behaves as expected but the caller is
>>>>>> wrong
>>>>>> because pfn_valid != RAM (this applies btw to !arm64 as well).
>>>>>>
>>>>>>      /* Don't allow RAM to be mapped */
>>>>>>      if (WARN_ON_ONCE(pfn_valid(PHYS_PFN(phys_addr))))
>>>>>>          return DMA_MAPPING_ERROR;
>>>>>>
>>>>>> Alex, can you please try this patch:
>>>>>
>>>>> That will certainly paper over the issue, but it's avoiding the
>>>>> question
>>>>> of what went wrong with the memory map in the first place. The comment
>>>>> is indeed a bit inaccurate, but ultimately dma_map_resource() exists
>>>>> for
>>>>> addresses that would be wrong to pass to dma_map_page(), so I believe
>>>>> pfn_valid() is still the correct check.
>>>>
>>>> If we want to check for RAM, pfn_valid() would be wrong. If we want
>>>> to check
>>>> for "is there a memmap, for whatever lives or does not live there",
>>>> pfn_valid() is the right check.
>>>
>>> So what should the DMA code use instead? Last time we needed something
>>> similar, the recommendation was to use pfn_to_online_page(). Mike is
>>> suggesting memblock_is_memory().
>>
>> We use pfn_to_online_page() when we want to know if it's system RAM and
>> that the memmap actually contains something sane (-> memmap content has
>> a well defined state).

Sorry that was only partially right: to be more precise 
pfn_to_online_page() might also succeeds on memory holes and similar 
(essentially everywhere where we don't have CONFIG_HAVE_ARCH_PFN_VALID), 
it just means that there is a memmap and that the memmap has a well 
defined. If it's system RAM, it's online and either getting used or 
lying around as free in the buddy.

For example, pfn_to_online_page() won't succeed on ZONE_DEVICE memory, 
while pfn_valid() will.


>>
>> You can have offline memory blocks where pfn_to_online_page() would
>> return "false", memblock_is_memory() would return "true". IOW, there is
>> a memmap, it's System RAM, but the memmap is stale and not trustworthy.
> 
> That's fine - offline memory is doubly-invalid to map as an MMIO resource :)
> 
>> If you want to make sure no System RAM (online/offline/...) will get
>> mapped, memblock_is_memory() should be the right thing to use. I recall
>> that x86 traverse the resource tree instead to exclude system ram
>> regions similarly.
> 
> I'm thinking that "pfn_valid(pfn) && !PageReserved(pfn_to_page(pfn))"

For offline memory blocks or for ZONE_DEVICE memory where pfn_valid() 
succeeds, the memmap might be stale and not contain anything reasonable. 
So the PageReserved() check can be problematic.

Also note that PageReserved() has plenty of different semantics (see 
PG_reserved in include/linux/page-flags.h )

> might be the closest thing to what I'd like to express - does that seem
> sensible at all? The one thing I'm not quite sure about is the
> interaction with P2P mappings of ZONE_DEVICE, but that's also true of
> the previous behaviour, and I'm not aware that the usage model has been
> fully nailed down yet, so I suspect we have some wiggle room. At worst,

Yes.

> false negatives in certain situations wouldn't be the end of the world,
> since this is merely a sanity check for something which is expected to
> be a no-op the vast majority of the time, so being unobtrusive is more
> important than being exhaustive.

Right.

-- 
Thanks,

David / dhildenb