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Re: framebuffer corruption due to overlapping stp instructions on arm64


On Wed, Aug 08, 2018 at 10:12:27AM -0400, 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.

It could get it wrong between subsequent writes to the same 64-bit range
(e.g. the address & ~63 is the same but the data strobes for which bytes
to write are different). If it somehow thinks that it's a
write-after-write hazard even though the strobes are different, it could
cancel one of the writes.

It may be worth trying with a byte-only memcpy() function while keeping
the default memcmp().

-- 
Catalin


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