• Joel Stanley's avatar
    powerpc/perf: Clear MMCR2 when enabling PMU · b50a6c58
    Joel Stanley authored
    On POWER8 when switching to a KVM guest we set bits in MMCR2 to freeze
    the PMU counters. Aside from on boot they are then never reset,
    resulting in stuck perf counters for any user in the guest or host.
    
    We now set MMCR2 to 0 whenever enabling the PMU, which provides a sane
    state for perf to use the PMU counters under either the guest or the
    host.
    
    This was manifesting as a bug with ppc64_cpu --frequency:
    
        $ sudo ppc64_cpu --frequency
        WARNING: couldn't run on cpu 0
        WARNING: couldn't run on cpu 8
          ...
        WARNING: couldn't run on cpu 144
        WARNING: couldn't run on cpu 152
        min:    18446744073.710 GHz (cpu -1)
        max:    0.000 GHz (cpu -1)
        avg:    0.000 GHz
    
    The command uses a perf counter to measure CPU cycles over a fixed
    amount of time, in order to approximate the frequency of the machine.
    The counters were returning zero once a guest was started, regardless of
    weather it was still running or had been shut down.
    
    By dumping the value of MMCR2, it was observed that once a guest is
    running MMCR2 is set to 1s - which stops counters from running:
    
        $ sudo sh -c 'echo p > /proc/sysrq-trigger'
        CPU: 0 PMU registers, ppmu = POWER8 n_counters = 6
        PMC1:  5b635e38 PMC2: 00000000 PMC3: 00000000 PMC4: 00000000
        PMC5:  1bf5a646 PMC6: 5793d378 PMC7: deadbeef PMC8: deadbeef
        MMCR0: 0000000080000000 MMCR1: 000000001e000000 MMCRA: 0000040000000000
        MMCR2: fffffffffffffc00 EBBHR: 0000000000000000
        EBBRR: 0000000000000000 BESCR: 0000000000000000
        SIAR:  00000000000a51cc SDAR:  c00000000fc40000 SIER:  0000000001000000
    
    This is done unconditionally in book3s_hv_interrupts.S upon entering the
    guest, and the original value is only save/restored if the host has
    indicated it was using the PMU. This is okay, however the user of the
    PMU needs to ensure that it is in a defined state when it starts using
    it.
    
    Fixes: e05b9b9e ("powerpc/perf: Power8 PMU support")
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarJoel Stanley <joel@jms.id.au>
    Acked-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
    b50a6c58
core-book3s.c 52.5 KB