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The idea is to get to the point where if r13 is non-zero, then it should contain a reasonable paca. This can be used in early boot program check and machine check handlers to avoid running off into the weeds if they hit before r13 has a paca. Signed-off-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://lore.kernel.org/r/20220926055620.2676869-5-npiggin@gmail.com