Created attachment 282743 [details]
kernel .config (PowerMac G4 MDD)

Trying out older kernels on the G4 MDD I noticed recent 4.9.x freeze the machine. Only message displayed in black letters on a white screen:

done
found display   : /pco@f0000000/ATY,AlteracParent@10/ATY,Alterac_B@1, opening...


It's a hard freeze, RCU_CPU_STALL_TIMEOUT does not kick in.

Tried other stable kernels, which all worked:
4.19.37
4.14.114 
4.4.179

So I suppose it's only a 4.9.x issue. Last working 4.9.x kernel I had in service was 4.9.161. First one I spotted freezing was 4.9.171. A bisect gave me the following commit:

1c38a84d45862be06ac418618981631eddbda741 is the first bad commit
commit 1c38a84d45862be06ac418618981631eddbda741
Author: Michael Neuling <mikey@neuling.org>
Date:   Thu Apr 11 21:45:59 2019 +1000

    powerpc: Avoid code patching freed init sections
    
    commit 51c3c62b58b357e8d35e4cc32f7b4ec907426fe3 upstream.
    
    This stops us from doing code patching in init sections after they've
    been freed.
    
    In this chain:
      kvm_guest_init() ->
        kvm_use_magic_page() ->
          fault_in_pages_readable() ->
             __get_user() ->
               __get_user_nocheck() ->
                 barrier_nospec();
    
    We have a code patching location at barrier_nospec() and
    kvm_guest_init() is an init function. This whole chain gets inlined,
    so when we free the init section (hence kvm_guest_init()), this code
    goes away and hence should no longer be patched.
    
    We seen this as userspace memory corruption when using a memory
    checker while doing partition migration testing on powervm (this
    starts the code patching post migration via
    /sys/kernel/mobility/migration). In theory, it could also happen when
    using /sys/kernel/debug/powerpc/barrier_nospec.
    
    Cc: stable@vger.kernel.org # 4.13+
    Signed-off-by: Michael Neuling <mikey@neuling.org>
    Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
    Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Signed-off-by: Sasha Levin <sashal@kernel.org>