Re: framebuffer corruption due to overlapping stp instructions on arm64

Catalin Marinas
2018-08-08T15:14:53Z

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:
> >>> while (1) {
> >>> start = (unsigned)random() % (LEN + 1);
> >>> end = (unsigned)random() % (LEN + 1);
> >>> if (start > end)
> >>> continue;
> >>> for (i = start; i < end; i++)
> >>> data[i] = val++;
> >>> memcpy(map + start, data + start, end - start);
> >>> if (memcmp(map, data, LEN)) {
> >>
> >> It may be worth trying to do a memcmp(map+start, data+start, end-start)
> >> here to see whether the hazard logic fails when the writes are unaligned
> >> but the reads are not.
> >>
> >> This problem may as well appear if you do byte writes and read longs
> >> back (and I consider this a hardware problem on this specific board).
> >
> > I triad to insert usleep(10000) between the memcpy and memcmp, but the
> > same corruption occurs. So, it can't be read-after-write hazard. It is
> > caused by the improper handling of hazard between the overlapping writes
> > inside memcpy.
>
> I don't think you've told us what form the corruption takes. Does it
> lose some bytes? Modify values beyond the copy range? Write completely
> arbitrary values?

From this message:

https://lore.kernel.org/lkml/alpine.LRH.2.02.1808060553130.30832@file01.intranet.prod.int.rdu2.redhat.com/

- 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).

--
Catalin