Re: framebuffer corruption due to overlapping stp instructions on arm64

Mikulas Patocka
2018-08-08T18:25:54Z

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/

> - What if we use a pair of str instructions instead of an stp in
> a modified memcpy? Does it now write to still write to the
> wrong place 16 bytes away, just 8 bytes away, or correctly?

I replaced all stp instructions with str and it didn't have effect on
corruptions. Either a few bytes is omitted, or a value that belongs 16
bytes before or after is written.

> - Does it change in any way if we do the overlapping writes
> in the reverse order? E.g. for the 16..64 byte case:
>
> diff --git a/sysdeps/aarch64/memcpy.S b/sysdeps/aarch64/memcpy.S
> index 7e1163e6a0..09d0160bdf 100644
> --- a/sysdeps/aarch64/memcpy.S
> +++ b/sysdeps/aarch64/memcpy.S
> @@ -102,11 +102,11 @@ ENTRY (MEMCPY)
> tbz tmp1, 5, 1f
> ldp B_l, B_h, [src, 16]
> ldp C_l, C_h, [srcend, -32]
> - stp B_l, B_h, [dstin, 16]
> stp C_l, C_h, [dstend, -32]
> + stp B_l, B_h, [dstin, 16]
> 1:
> - stp A_l, A_h, [dstin]
> stp D_l, D_h, [dstend, -16]
> + stp A_l, A_h, [dstin]
> ret
>
> .p2align 4
>
> Arnd

After reordering them, I observe only omitted writes, there are no longer
misdirected writes:

http://people.redhat.com/~mpatocka/testcases/arm-pcie-corruption/reorder-test/

Mikulas