On Wed, 8 Aug 2018, Arnd Bergmann wrote: > On Wed, Aug 8, 2018 at 5:15 PM Catalin Marinas <catalin.marinas@xxxxxxx> 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
