• Michael Ellerman's avatar
    powerpc/kvm: Fix lockups when running KVM guests on Power8 · 56376c58
    Michael Ellerman authored
    When running KVM guests on Power8 we can see a lockup where one CPU
    stops responding. This often leads to a message such as:
    
      watchdog: CPU 136 detected hard LOCKUP on other CPUs 72
      Task dump for CPU 72:
      qemu-system-ppc R  running task    10560 20917  20908 0x00040004
    
    And then backtraces on other CPUs, such as:
    
      Task dump for CPU 48:
      ksmd            R  running task    10032  1519      2 0x00000804
      Call Trace:
        ...
        --- interrupt: 901 at smp_call_function_many+0x3c8/0x460
            LR = smp_call_function_many+0x37c/0x460
        pmdp_invalidate+0x100/0x1b0
        __split_huge_pmd+0x52c/0xdb0
        try_to_unmap_one+0x764/0x8b0
        rmap_walk_anon+0x15c/0x370
        try_to_unmap+0xb4/0x170
        split_huge_page_to_list+0x148/0xa30
        try_to_merge_one_page+0xc8/0x990
        try_to_merge_with_ksm_page+0x74/0xf0
        ksm_scan_thread+0x10ec/0x1ac0
        kthread+0x160/0x1a0
        ret_from_kernel_thread+0x5c/0x78
    
    This is caused by commit 8c1c7fb0 ("powerpc/64s/idle: avoid sync
    for KVM state when waking from idle"), which added a check in
    pnv_powersave_wakeup() to see if the kvm_hstate.hwthread_state is
    already set to KVM_HWTHREAD_IN_KERNEL, and if so to skip the store and
    test of kvm_hstate.hwthread_req.
    
    The problem is that the primary does not set KVM_HWTHREAD_IN_KVM when
    entering the guest, so it can then come out to cede with
    KVM_HWTHREAD_IN_KERNEL set. It can then go idle in kvm_do_nap after
    setting hwthread_req to 1, but because hwthread_state is still
    KVM_HWTHREAD_IN_KERNEL we will skip the test of hwthread_req when we
    wake up from idle and won't go to kvm_start_guest. From there the
    thread will return somewhere garbage and crash.
    
    Fix it by skipping the store of hwthread_state, but not the test of
    hwthread_req, when coming out of idle. It's OK to skip the sync in
    that case because hwthread_req will have been set on the same thread,
    so there is no synchronisation required.
    
    Fixes: 8c1c7fb0 ("powerpc/64s/idle: avoid sync for KVM state when waking from idle")
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    56376c58
idle_book3s.S 25.9 KB