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SubjectRE: framebuffer corruption due to overlapping stp instructions on arm64
From: Arnd Bergmann
> Sent: 08 August 2018 17:31
> > They do modify the same byte, but with the same value. Suppose that you
> > want to copy a piece of data that is between 8 and 16 bytes long. You can
> > do this:
> >
> > add src_end, src, len
> > add dst_end, dst, len
> > ldr x0, [src]
> > ldr x1, [src_end - 8]
> > str x0, [dst]
> > str x1, [dst_end - 8]

I've done that myself (on x86) copied the last 'word' first then
everything else in increasing address order.

> > The ARM64 memcpy uses this trick heavily in order to reduce branching, and
> > this is what makes the PCIe controller choke.

More likely the write combining buffer?

> So when a single unaligned 'stp' gets translated into a PCIe with TLP
> with length=5 (20 bytes) and LastBE = ~1stBE, write combining the
> overlapping stores gives us a TLP with a longer length (5..8 for two
> stores), and byte-enable bits that are not exactly a complement.

Write combining should generate a much longer TLP.
Depending on the size of the write combining buffer.

But in the above case I'd have thought that the second write
would fail to 'combine' - because it isn't contiguous with the
stored data.

So something more complex will be going on.


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