Re: framebuffer corruption due to overlapping stp instructions on arm64

Arnd Bergmann
2018-08-08T21:52:14Z

On Wed, Aug 8, 2018 at 8:25 PM Mikulas Patocka <mpatocka@redhat.com> wrote:
> On Wed, 8 Aug 2018, Arnd Bergmann wrote:
>
> > On Wed, Aug 8, 2018 at 5:15 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
> > >
> > > On Wed, Aug 08, 2018 at 04:01:12PM +0100, Richard Earnshaw wrote:
> > > > On 08/08/18 15:12, Mikulas Patocka wrote:
> > > > > On Wed, 8 Aug 2018, Catalin Marinas wrote:
> > > > >> On Fri, Aug 03, 2018 at 01:09:02PM -0400, Mikulas Patocka wrote:
> > > - failing to write a few bytes
> > > - writing a few bytes that were written 16 bytes before
> > > - writing a few bytes that were written 16 bytes after
> > >
> > > > The overlapping writes in memcpy never write different values to the
> > > > same location, so I still feel this must be some sort of HW issue, not a
> > > > SW one.
> > >
> > > So do I (my interpretation is that it combines or rather skips some of
> > > the writes to the same 16-byte address as it ignores the data strobes).
> >
> > Maybe it just always writes to the wrong location, 16 bytes apart for one of
> > the stp instructions. Since we are usually dealing with a pair of overlapping
> > 'stp', both unaligned, that could explain both the missing bytes (we write
> > data to the wrong place, but overwrite it with the correct data right away)
> > and the extra copy (we write it to the wrong place, but then write the correct
> > data to the correct place as well).
> >
> > This sounds a bit like what the original ARM CPUs did on unaligned
> > memory access, where a single aligned 4-byte location was accessed,
> > but the bytes swapped around.
> >
> > There may be a few more things worth trying out or analysing from
> > the recorded past failures to understand more about how it goes
> > wrong:
> >
> > - For which data lengths does it fail? Having two overlapping
> > unaligned stp is something that only happens for 16..96 byte
> > memcpy.
>
> If you want to research the corruptions in detail, I uploaded a file
> containing 7k corruptions here:
> http://people.redhat.com/~mpatocka/testcases/arm-pcie-corruption/

Nice!

I already found a couple of things:

- Failure to copy always happens at the *end* of a 16 byte aligned
physical address, it misses between 1 and 6 bytes, never 7 or more,
and it's more likely to be fewer bytes that are affected.
279 7
389 6
484 5
683 4
741 3
836 2
946 1

- The first byte that fails to get copied is always 16 bytes after the
memcpy target. Since we only observe it at the end of the 16 byte
range, it means this happens specifically for addresses ending in
0x9 (7 bytes missed) to 0xf (1 byte missed).

- Out of 7445 corruptions, 4358 were of the kind that misses a copy at the
end of a 16-byte area, they were for copies between 41 and 64 bytes,
more to the larger end of the scale (note that with your test program,
smaller memcpys happen more frequenly than larger ones).
47 0x29
36 0x2a
47 0x2b
23 0x2c
29 0x2d
31 0x2e
36 0x2f
46 0x30
45 0x31
51 0x32
62 0x33
64 0x34
77 0x35
91 0x36
90 0x37
100 0x38
100 0x39
209 0x3a
279 0x3b
366 0x3c
498 0x3d
602 0x3e
682 0x3f
747 0x40

- All corruption with data copied to the wrong place happened for copies
between 33 and 47 bytes, mostly to the smaller end of the scale:
391 0x21
360 0x22
319 0x23
273 0x24
273 0x25
241 0x26
224 0x27
221 0x28
231 0x29
208 0x2a
163 0x2b
86 0x2c
63 0x2d
33 0x2e
1 0x2f

- One common (but not the only, still investigating) case for data getting
written to the wrong place is:
* corruption starts 16 bytes after the memcpy start
* corrupt bytes are the same as the bytes written to the start
* start address ends in 0x1 through 0x7
* length of corruption is at most memcpy length- 32, always
between 1 and 7.

Arnd