Messages in this thread | | | Date | Wed, 28 Dec 2016 23:43:22 +0000 | From | Al Viro <> | Subject | Re: [PATCH v2 1/3] introduce memcpy_nocache() |
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On Tue, Nov 01, 2016 at 04:25:12PM +0200, Boaz Harrosh wrote:
> >> What about memcpy_to_pmem() in linux/pmem.h it already has all the arch switches. > >> > >> Feels bad to add yet just another arch switch over __copy_user_nocache > >> > >> Just feels like too many things that do the same thing. Sigh > > > > I agree that this looks like a nicer path. > > > > I had considered adjusting copy_from_iter_nocache() to use memcpy_to_pmem(), > > but lib/iov_iter.c doesn't currently #include linux/pmem.h. Would it be > > acceptable to add it? Also, I wasn't sure if memcpy_to_pmem() would always > > mean exactly "memcpy nocache". > > > > I think this is the way to go. In my opinion there is no reason why not to include > pmem.h into lib/iov_iter.c. > > And I think memcpy_to_pmem() would always be the fastest arch way to bypass cache > so it should be safe to use this for all cases. It is so in the arches that support > this now, and I cannot imagine a theoretical arch that would differ. But let the > specific arch people holler if this steps on their tows, later when they care about > this at all.
First of all, if it's the fastest arch way to bypass cache, why the hell is it sitting in pmem-related areas?
More to the point, x86 implementation of that thing is tied to uaccess API for no damn reason whatsoever. Let's add a real memcpy_nocache() and be done with that. I mean, this if (WARN(rem, "%s: fault copying %p <- %p unwritten: %d\n", __func__, dst, src, rem)) BUG(); is *screaming* "API misused here". And let's stay away from the STAC et.al. - it's pointless for kernel-to-kernel copies.
BTW, your "it's iovec, only non-temporal stores there" logics in arch_copy_from_iter_pmem() is simply wrong - for one thing, unaligned copies will have parts done via normal stores, for another 32bit will _not_ go for non-caching codepath for short copies. What semantics do we really need there?
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