On Mon, Jul 22, 2019 at 08:15:14PM +1000, Michael Ellerman wrote: > Segher Boessenkool <seg...@kernel.crashing.org> writes: > > On Sun, Jul 21, 2019 at 12:58:46AM -0700, Nathan Chancellor wrote: > >> 0000017c clear_user_page: > >> 17c: 94 21 ff f0 stwu 1, -16(1) > >> 180: 38 80 00 80 li 4, 128 > >> 184: 38 63 ff e0 addi 3, 3, -32 > >> 188: 7c 89 03 a6 mtctr 4 > >> 18c: 38 81 00 0f addi 4, 1, 15 > >> 190: 8c c3 00 20 lbzu 6, 32(3) > >> 194: 98 c1 00 0f stb 6, 15(1) > >> 198: 7c 00 27 ec dcbz 0, 4 > >> 19c: 42 00 ff f4 bdnz .+65524 > > > > Uh, yeah, well, I have no idea what clang tried here, but that won't > > work. It's copying a byte from each target cache line to the stack, > > and then does clears the cache line containing that byte on the stack. > > So it seems like this is a clang bug. > > None of the distros we support use clang, but we would still like to > keep it working if we can.
Which version? Which versions *are* broken? > Looking at the original patch, the only upside is that the compiler > can use both RA and RB to compute the address, rather than us forcing RA > to 0. > > But at least with my compiler here (GCC 8 vintage) I don't actually see > GCC ever using both GPRs even with the patch. Or at least, there's no > difference before/after the patch as far as I can see. The benefit is small, certainly. > So my inclination is to revert the original patch. We can try again in a > few years :D > > Thoughts? I think you should give the clang people time to figure out what is going on. Segher
