Re: framebuffer corruption due to overlapping stp instructions on arm64

[Ard Biesheuvel]

On 3 August 2018 at 22:44, Matt Sealey <neko@xxxxxxxxxxxxx> wrote:
> On 3 August 2018 at 13:25, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote:
>>
>>
>> On Fri, 3 Aug 2018, Ard Biesheuvel wrote:
>>
>>> Are we still talking about overlapping unaligned accesses here? Or do
>>> you see other failures as well?
>>
>> Yes - it is caused by overlapping unaligned accesses inside memcpy. When I
>> put "dmb sy" between the overlapping accesses in
>> glibc/sysdeps/aarch64/memcpy.S, this program doesn't detect any memory
>> corruption.
>
> It is a symptom of generating reorderable accesses inside memcpy. It's nothing
> to do with alignment, per se (see below). A dmb sy just hides the symptoms.
>
> What we're talking about here - yes, Ard, within certain amounts of
> reason - is that
> you cannot use PCI BAR memory as 'Normal' - certainly never cacheable memory,
> but Normal NC isn't good either. That is that your CPU cannot post
> writes or reads
> towards PCI memory spaces unless it is dealing with it as Device memory or very
> strictly controlled use of Normal Non-Cacheable.
>
> I understand why the rest of the world likes to mark stuff as
> 'writecombine,' but
> that's x86-ism, not an Arm memory type.
>
> There is potential for accesses to the same slave from different
> masters (or just
> different AXI IDs, most cores rotate over 8 or 16 or so for Normal
> memory to achieve)
> to be reordered. PCIe has no idea what the source was, it will just
> accept them in the order it receives them, and also it will be
> strictly defined to
> manage incoming AXI or ACE transactions (and barriers..) in a way that does
> not violate the PCIe memory model - the worst case is deadlocks, the best case
> is you see some very strange behavior.
>
> In any case the original ordering of two Normal-NC transactions may
> not make it to
> the PCIe bridge in the first place which is probably why a DMB
> resolves it - it will
> force the core to issue them in order and it's likely unless there is
> some hyper-complex
> multi-pathing going on, they'll stay ordered. If you MUST preserve the
> order between
> two Normal memory accesses, a barrier is required. The same is true also of any
> re-orderable device access.
>

None of this explains why some transactions fail to make it across
entirely. The overlapping writes in question write the same data to
the memory locations that are covered by both, and so the ordering in
which the transactions are received should not affect the outcome.



>>> > I tried to run it on system RAM mapped with the NC attribute and I didn't
>>> > get any corruption - that suggests the the bug may be in the PCIE
>>> > subsystem.
>
> Pure fluke.
>
> I'll give a simple explanation. The Arm Architecture defines
> single-copy and multi-copy
> atomic transactions. You can treat 'single-copy' to mean that that
> transaction cannot
> be made partial, or reordered within itself, i.e. it must modify
> memory (if it is a store) in
> a single swift effort and any future reads from that memory must
> return the FULL result
> of that write.
>
> Multi-copy means it can be resized and reordered a bit. Will Deacon is
> going to crucify
> me for simplifying it, but.. let's proceed with a poor example:
>
> STR X0,[X1] on a 32-bit bus cannot ever be single-copy atomic, because
> you cannot
> write 64-bits of data on a 32-bit bus in a single, unbreakable
> transaction. This is because
> from one bus cycle to the next, one half of the transaction will be in
> a different place. Your
> interconnect will have latched and buffered 32-bits and the CPU is
> holding the other.
>
> STP X0, X1, [X2] on a 64-bit bus can be single-copy atomic with
> respect to the element
> size. But it is on the whole multi-copy atomic - that is to say that
> it can provide a single
> transaction with multiple elements which are transmitted, and those
> elements could be
> messed with on the way down the pipe.
>
> On a 128-bit bus, you might expect it to be single-copy atomic because
> the entire
> transaction can be fit into one single data beat, but *it is most
> definitely not* according
> to the architecture. The data from X0 and X1 may be required to be
> stored at *X2 and
> *(X2+8), but the architecture doesn't care which one is written first.
> Neither does AMBA.
>
> STP is only ever guaranteed to be single-copy atomic with regards to
> the element size
> (which is the X register in question). If you swap the data around,
> and do STP X1, X0,
> [X2] you may see a different result dependent on how the processor
> decides to pull
> data from the register file and in what order. Users of the old 32-bit
> ARM STM instruction
> will recall that it writes the register list in incrementing order,
> lowest register number to
> lowest address, so what is the solution for STP? Do you expect expect
> X0 to be emitted
> on the bus first or the data to be stored in *X2?
>
> It's neither!
>
> That means you can do an STP on one processor and an LDR of one of the 64-bit
> words on another processor, and you may be able to see
>
> a) None of the STP transaction
> b) X2 is written with the value in X0, but X2+8 is not holding the value in X1
> c) b, only reversed
> d) What you expect
>
> And this can change dependent on the resizers and bridges and QoS and paths
> between a master interface and a slave interface, although a truly
> single-copy atomic
> transaction going through a downsizer to smaller than the transaction
> size is a broken
> system design, it may be allowable if the downsizer hazards addresses
> to the granularity
> of the larger bus size on the read and write channels and will stall
> the read until the write
> has committed at least to a buffer, or downstream of the downsizer, so
> that it will return
> on read the full breadth of the memory update.... that's down to the
> system designer.
> There are plenty of places things like this can happen - in cache
> controllers, for
> example, and merging store buffers (you may have a 256 bit or 512 bit
> buffer, but
> only a 128-bit memory interface).
>
> memcpy() as a function nor the loads and stores it makes are not
> single-copy atomic,
> no transactions need to be with Normal memory, so that merged stores
> and linefills
> (if cacheable) can be done. Hence, your memcpy() is just randomly
> chucking whatever
> data it likes to the bus and they'll arrive in any old order,
> 'writecombine' semantics make
> you think you'll only ever see one very large write with all the CPU
> activity merged
> together - also NOT true.
>
> And the granularity of the hazarding in your system, from the CPU
> store buffer to the
> bus interface to the interconnect buffering to the PCIe bridge to the
> PCIe EP is.. what?
> Not the same all the way down, I'll bet you.
>
> It is assuming that Intel writecombine semantics would apply, which to
> be truthful are NO
> different to the ones of a merging store buffer in an Arm processor
> (Intel architecture states
> that the writecombine buffer can be flushed at any time with any
> amount of actual data,
> it might not be the biggest burst you can imagine), but in practice it
> tends to be in cache-line
> sized chunks with strict incrementing order and subsequent writes due
> to the extremely
> large pipeline and queueing will be absorbed by the writecombine
> buffer almost with
> guarantee.
>
> Links is broken. Even on Intel. If you overlap memory transactions and
> expect them to be
> gathered and reordered to produce nice, ordered non-overlapping
> streaming transactions
> you'll be sorely disappointed when they don't, which is what is
> happening here. The fix is
> use barriers - and don't rely on single-copy atomicity (which is the
> only saving feature that
> would not require you to use a barrier) since this is a situation
> where absolutely none is
> afforded.
>
> It'd be easier to cross your fingers that the PCIe RC is has a
> coherent master port (ACE-Lite
> or something fancier) and can snoop into CPU caches. Then you can mark a memory
> location in DRAM as Normal Inner/Outer Cacheable Writeback,
> Inner/Outer Shareable,
> Write-allocate, read-allocate, and you won't even notice your CPU
> doing any memory
> writes, but yes if you tell a graphics adapter that it's main
> framebuffer is in DRAM it might
> be a bit slower (to the speed of the PCIe link.. which may affect your
> maximum resolution
> in some really strange circumstances). If it cannot use a DRAM
> framebuffer then I'd have to
> wonder why not.. every PCI graphics card I ever used could take any
> base address and
> the magic of PCI bus mastering would handle it. This is no different
> to how you'd use
> DRAM as texture memory.. phenomenally slowly, but without having to
> worry about any
> ordering semantics (except you should flush your data cache to PoC at
> the end of every
> frame).
>
> Ta,
> Matt