• Lennert Buytenhek's avatar
    [ARM] alternative copy_to_user/clear_user implementation · 39ec58f3
    Lennert Buytenhek authored
    This implements {copy_to,clear}_user() by faulting in the userland
    pages and then using the regular kernel mem{cpy,set}() to copy the
    data (while holding the page table lock).  This is a win if the regular
    mem{cpy,set}() implementations are faster than the user copy functions,
    which is the case e.g. on Feroceon, where 8-word STMs (which memcpy()
    uses under the right conditions) give significantly higher memory write
    throughput than a sequence of individual 32bit stores.
    Here are numbers for page sized buffers on some Feroceon cores:
     - copy_to_user on Orion5x goes from 51 MB/s to 83 MB/s
     - clear_user on Orion5x goes from 89MB/s to 314MB/s
     - copy_to_user on Kirkwood goes from 240 MB/s to 356 MB/s
     - clear_user on Kirkwood goes from 367 MB/s to 1108 MB/s
     - copy_to_user on Disco-Duo goes from 248 MB/s to 398 MB/s
     - clear_user on Disco-Duo goes from 328 MB/s to 1741 MB/s
    Because the setup cost is non negligible, this is worthwhile only if
    the amount of data to copy is large enough.  The operation falls back
    to the standard implementation when the amount of data is below a certain
    threshold. This threshold was determined empirically, however some targets
    could benefit from a lower runtime determined value for optimal results
    In the copy_from_user() case, this technique does not provide any
    worthwhile performance gain due to the fact that any kind of read access
    allocates the cache and subsequent 32bit loads are just as fast as the
    equivalent 8-word LDM.
    Signed-off-by: default avatarLennert Buytenhek <buytenh@marvell.com>
    Signed-off-by: default avatarNicolas Pitre <nico@marvell.com>
    Tested-by: default avatarMartin Michlmayr <tbm@cyrius.com>
Kconfig 36.8 KB