On Fri, May 5, 2017 at 1:39 PM, Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote: > On Fri, 2017-04-28 at 12:39 -0700, Dan Williams wrote: >> The pmem driver has a need to transfer data with a persistent memory >> destination and be able to rely on the fact that the destination >> writes are not cached. It is sufficient for the writes to be flushed >> to a cpu-store-buffer (non-temporal / "movnt" in x86 terms), as we >> expect userspace to call fsync() to ensure data-writes have reached a >> power-fail-safe zone in the platform. The fsync() triggers a REQ_FUA >> or REQ_FLUSH to the pmem driver which will turn around and fence >> previous writes with an "sfence". >> >> Implement a __copy_from_user_inatomic_wt, memcpy_page_wt, and >> memcpy_wt, that guarantee that the destination buffer is not dirty in >> the cpu cache on completion. The new copy_from_iter_wt and sub- >> routines will be used to replace the "pmem api" (include/linux/pmem.h >> + arch/x86/include/asm/pmem.h). The availability of >> copy_from_iter_wt() and memcpy_wt() are gated by the >> CONFIG_ARCH_HAS_UACCESS_WT config symbol, and fallback to >> copy_from_iter_nocache() and plain memcpy() otherwise. >> >> This is meant to satisfy the concern from Linus that if a driver >> wants to do something beyond the normal nocache semantics it should >> be something private to that driver [1], and Al's concern that >> anything uaccess related belongs with the rest of the uaccess code >> [2]. >> >> [1]: https://lists.01.org/pipermail/linux-nvdimm/2017-January/008364. >> html >> [2]: https://lists.01.org/pipermail/linux-nvdimm/2017-April/009942.ht >> ml >> >> Cc: <x86@xxxxxxxxxx> >> Cc: Jan Kara <jack@xxxxxxx> >> Cc: Jeff Moyer <jmoyer@xxxxxxxxxx> >> Cc: Ingo Molnar <mingo@xxxxxxxxxx> >> Cc: Christoph Hellwig <hch@xxxxxx> >> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> >> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> >> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> >> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> >> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> >> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> >> --- >> Changes since the initial RFC: >> * s/writethru/wt/ since we already have ioremap_wt(), >> set_memory_wt(), etc. (Ingo) > > Sorry I should have said earlier, but I think the term "wt" is > misleading. Non-temporal stores used in memcpy_wt() provide WC > semantics, not WT semantics. The non-temporal stores do, but memcpy_wt() is using a combination of non-temporal stores and explicit cache flushing. > How about using "nocache" as it's been > used in __copy_user_nocache()? The difference in my mind is that the "_nocache" suffix indicates opportunistic / optional cache pollution avoidance whereas "_wt" strictly arranges for caches not to contain dirty data upon completion of the routine. For example, non-temporal stores on older x86 cpus could potentially leave dirty data in the cache, so memcpy_wt on those cpus would need to use explicit cache flushing.
