Messages in this thread | | | Date | Wed, 4 Jan 2017 01:59:12 +0000 | From | Al Viro <> | Subject | Re: [RFC] memcpy_nocache() and memcpy_writethrough() |
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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.
> > In particular, is that sfence the right thing for pmem usecases? > > That sfence is not there for pmem purposes. The dax / pmem usage does > not expect memcpy_to_pmem() to fence as it may have more writes to > queue up and amortize all the writes with a later fence. This seems to > be even more evidence for moving this functionality away from the > uaccess routines to somewhere more pmem specific.
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