• Dan Williams's avatar
    x86, uaccess: introduce copy_from_iter_flushcache for pmem / cache-bypass operations · 0aed55af
    Dan Williams authored
    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_flushcache, memcpy_page_flushcache, and
    memcpy_flushcache, that guarantee that the destination buffer is not dirty in
    the cpu cache on completion. The new copy_from_iter_flushcache 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_flushcache()
    and memcpy_flushcache() are gated by the CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
    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].
    
    The first consumer of this interface is a new 'copy_from_iter' dax operation so
    that pmem can inject cache maintenance operations without imposing this
    overhead on other dax-capable drivers.
    
    [1]: https://lists.01.org/pipermail/linux-nvdimm/2017-January/008364.html
    [2]: https://lists.01.org/pipermail/linux-nvdimm/2017-April/009942.html
    
    Cc: <x86@kernel.org>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Jeff Moyer <jmoyer@redhat.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Christoph Hellwig <hch@lst.de>
    Cc: Toshi Kani <toshi.kani@hpe.com>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Matthew Wilcox <mawilcox@microsoft.com>
    Reviewed-by: default avatarRoss Zwisler <ross.zwisler@linux.intel.com>
    Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
    0aed55af
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