• Paul Mackerras's avatar
    powerpc: Revert "Use the POWER8 Micro Partition Prefetch Engine in KVM HV on POWER8" · 23316316
    Paul Mackerras authored
    This reverts commit 9678cdaa ("Use the POWER8 Micro Partition
    Prefetch Engine in KVM HV on POWER8") because the original commit had
    multiple, partly self-cancelling bugs, that could cause occasional
    memory corruption.
    
    In fact the logmpp instruction was incorrectly using register r0 as the
    source of the buffer address and operation code, and depending on what
    was in r0, it would either do nothing or corrupt the 64k page pointed to
    by r0.
    
    The logmpp instruction encoding and the operation code definitions could
    be corrected, but then there is the problem that there is no clearly
    defined way to know when the hardware has finished writing to the
    buffer.
    
    The original commit attempted to work around this by aborting the
    write-out before starting the prefetch, but this is ineffective in the
    case where the virtual core is now executing on a different physical
    core from the one where the write-out was initiated.
    
    These problems plus advice from the hardware designers not to use the
    function (since the measured performance improvement from using the
    feature was actually mostly negative), mean that reverting the code is
    the best option.
    
    Fixes: 9678cdaa ("Use the POWER8 Micro Partition Prefetch Engine in KVM HV on POWER8")
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    23316316
reg.h 55.3 KB