• Paul Mackerras's avatar
    powerpc: Fix "attempt to move .org backwards" error · a485c709
    Paul Mackerras authored
    Building a 64-bit powerpc kernel with PR KVM enabled currently gives
    this error:
      AS      arch/powerpc/kernel/head_64.o
    arch/powerpc/kernel/exceptions-64s.S: Assembler messages:
    arch/powerpc/kernel/exceptions-64s.S:258: Error: attempt to move .org backwards
    make[2]: *** [arch/powerpc/kernel/head_64.o] Error 1
    This happens because the MASKABLE_EXCEPTION_PSERIES macro turns into
    33 instructions, but we only have space for 32 at the decrementer
    interrupt vector (from 0x900 to 0x980).
    In the code generated by the MASKABLE_EXCEPTION_PSERIES macro, we
    currently have two instances of the HMT_MEDIUM macro, which has the
    effect of setting the SMT thread priority to medium.  One is the
    first instruction, and is overwritten by a no-op on processors where
    we save the PPR (processor priority register), that is, POWER7 or
    later.  The other is after we have saved the PPR.
    In order to reduce the code at 0x900 by one instruction, we omit the
    first HMT_MEDIUM.  On processors without SMT this will have no effect
    since HMT_MEDIUM is a no-op there.  On POWER5 and RS64 machines this
    will mean that the first few instructions take a little longer in the
    case where a decrementer interrupt occurs when the hardware thread is
    running at low SMT priority.  On POWER6 and later machines, the
    hardware automatically boosts the thread priority when a decrementer
    interrupt is taken if the thread priority was below medium, so this
    change won't make any difference.
    The alternative would be to branch out of line after saving the CFAR.
    However, that would incur an extra overhead on all processors, whereas
    the approach adopted here only adds overhead on older threaded processors.
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>