Re: [PATCH v2] x86, uaccess: introduce copy_from_iter_wt for pmem / writethrough operations

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(a)kernel.org&gt; Cc: Jan Kara <jack(a)suse.cz&gt; Cc: Jeff Moyer <jmoyer(a)redhat.com&gt; Cc: Ingo Molnar <mingo(a)redhat.com&gt; Cc: Christoph Hellwig <hch(a)lst.de&gt; Cc: "H. Peter Anvin" <hpa(a)zytor.com&gt; Cc: Al Viro <viro(a)zeniv.linux.org.uk&gt; Cc: Thomas Gleixner <tglx(a)linutronix.de&gt; Cc: Matthew Wilcox <mawilcox(a)microsoft.com&gt; Cc: Ross Zwisler <ross.zwisler(a)linux.intel.com&gt; Signed-off-by: Dan Williams <dan.j.williams(a)intel.com&gt; --- Changes since the initial RFC: * s/writethru/wt/ since we already have ioremap_wt(), set_memory_wt(), etc. (Ingo)