• Michael Ellerman's avatar
    powerpc/64s: Fix CPU_FTRS_ALWAYS vs DT CPU features · 81b654c2
    Michael Ellerman authored
    The cpu_has_feature() mechanism has an optimisation where at build
    time we construct a mask of the CPU feature bits that will always be
    true for the given .config, based on the platform/bitness/etc. that we
    are building for.
    
    That is incompatible with DT CPU features, where the set of CPU
    features is dependent on feature flags that are given to us by
    firmware.
    
    The result is that some feature bits can not be *disabled* by DT CPU
    features. Or more accurately, they can be disabled but they will still
    appear in the ALWAYS mask, meaning cpu_has_feature() will always
    return true for them.
    
    In the past this hasn't really been a problem because on Book3S
    64 (where we support DT CPU features), the set of ALWAYS bits has been
    very small. That was because we always built for POWER4 and later,
    meaning the set of common bits was small.
    
    The only bit that could be cleared by DT CPU features that was also in
    the ALWAYS mask was CPU_FTR_NODSISRALIGN, and that was only used in
    the alignment handler to create a fake DSISR. That code was itself
    deleted in 31bfdb03 ("powerpc: Use instruction emulation
    infrastructure to handle alignment faults") (Sep 2017).
    
    However the set of ALWAYS features changed with the recent commit
    db5ae1c1 ("powerpc/64s: Refine feature sets for little endian
    builds") which restricted the set of feature flags when building
    little endian to Power7 or later. That caused the ALWAYS mask to
    become much larger for little endian builds.
    
    The result is that the following feature bits can currently not
    be *disabled* by DT CPU features:
    
      CPU_FTR_REAL_LE, CPU_FTR_MMCRA, CPU_FTR_CTRL, CPU_FTR_SMT,
      CPU_FTR_PURR, CPU_FTR_SPURR, CPU_FTR_DSCR, CPU_FTR_PKEY,
      CPU_FTR_VMX_COPY, CPU_FTR_CFAR, CPU_FTR_HAS_PPR.
    
    To fix it we need to mask the set of ALWAYS features with the base set
    of DT CPU features, ie. the features that are always enabled by DT CPU
    features. That way there are no bits in the ALWAYS mask that are not
    also always set by DT CPU features.
    
    Fixes: db5ae1c1 ("powerpc/64s: Refine feature sets for little endian builds")
    Reviewed-by: default avatarNicholas Piggin <npiggin@gmail.com>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    81b654c2
cputable.h 23.3 KB