On Tue, Jan 3, 2017 at 5:59 PM, Al Viro <v...@zeniv.linux.org.uk> wrote: > On Tue, Jan 03, 2017 at 05:38:54PM -0800, Dan Williams wrote: >> > 1) memcpy_to_pmem() seems to rely upon the __copy_from_user_nocache() >> > having only used movnt; it does not attempt clwb at all. >> >> Yes, and there was a fix a while back to make sure it always used >> movnt so clwb after the fact is not required: >> >> a82eee742452 x86/uaccess/64: Handle the caching of 4-byte nocache >> copies properly in __copy_user_nocache() >> >> > 2) __copy_from_user_nocache() for short copies does not use movnt at all. >> > In that case neither sfence nor clwb is issued. >> >> For the 32bit case, yes, but the pmem driver should warn about this >> when it checks platform persistent memory capabilities (i.e. x86 32bit >> not supported). Ugh, we may have lost that warning for this specific >> case recently, I'll go double check and fix it up. >> >> > 3) it uses movnt only for part of copying in case of misaligned copy; >> > No clwb is issued, but sfence *is* - at the very end in 64bit case, >> > between movnt and copying the tail - in 32bit one. Incidentally, >> > while 64bit case takes care to align the destination for movnt part, >> > 32bit one does not. >> > >> > How much of the above is broken and what do the callers rely upon? >> >> 32bit issues are known, but 64bit path is ok since that fix above. > > Bollocks. That fix above does *NOT* eliminate all cached stores. Just look > at the damn function - it still does cached stores for until the target is > aligned and it does the same for tail when end of destination is not aligned. > Right there in arch/x86/lib/copy_user_64.S.
No, it does not eliminate all cache stores, but the cases where we use it have naturally aligned targets. Yes, it is terrible to then call wrap it in a memcpy_to_pmem() wrapper which does not document these alignment constraints. _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm