• Anton Blanchard's avatar
    powerpc/perf: Fix book3s kernel to userspace backtraces · 28aab9ee
    Anton Blanchard authored
    commit 72e349f1 upstream.
    
    When we take a PMU exception or a software event we call
    perf_read_regs(). This overloads regs->result with a boolean that
    describes if we should use the sampled instruction address register
    (SIAR) or the regs.
    
    If the exception is in kernel, we start with the kernel regs and
    backtrace through the kernel stack. At this point we switch to the
    userspace regs and backtrace the user stack with perf_callchain_user().
    
    Unfortunately these regs have not got the perf_read_regs() treatment,
    so regs->result could be anything. If it is non zero,
    perf_instruction_pointer() decides to use the SIAR, and we get issues
    like this:
    
    0.11%  qemu-system-ppc  [kernel.kallsyms]        [k] _raw_spin_lock_irqsave
           |
           ---_raw_spin_lock_irqsave
              |
              |--52.35%-- 0
              |          |
              |          |--46.39%-- __hrtimer_start_range_ns
              |          |          kvmppc_run_core
              |          |          kvmppc_vcpu_run_hv
              |          |          kvmppc_vcpu_run
              |          |          kvm_arch_vcpu_ioctl_run
              |          |          kvm_vcpu_ioctl
              |          |          do_vfs_ioctl
              |          |          sys_ioctl
              |          |          system_call
              |          |          |
              |          |          |--67.08%-- _raw_spin_lock_irqsave <--- hi mum
              |          |          |          |
              |          |          |           --100.00%-- 0x7e714
              |          |          |                     0x7e714
    
    Notice the bogus _raw_spin_irqsave when we transition from kernel
    (system_call) to userspace (0x7e714). We inserted what was in the SIAR.
    
    Add a check in regs_use_siar() to check that the regs in question
    are from a PMU exception. With this fix the backtrace makes sense:
    
         0.47%  qemu-system-ppc  [kernel.vmlinux]         [k] _raw_spin_lock_irqsave
                |
                ---_raw_spin_lock_irqsave
                   |
                   |--53.83%-- 0
                   |          |
                   |          |--44.73%-- hrtimer_try_to_cancel
                   |          |          kvmppc_start_thread
                   |          |          kvmppc_run_core
                   |          |          kvmppc_vcpu_run_hv
                   |          |          kvmppc_vcpu_run
                   |          |          kvm_arch_vcpu_ioctl_run
                   |          |          kvm_vcpu_ioctl
                   |          |          do_vfs_ioctl
                   |          |          sys_ioctl
                   |          |          system_call
                   |          |          __ioctl
                   |          |          0x7e714
                   |          |          0x7e714
    Signed-off-by: default avatarAnton Blanchard <anton@samba.org>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    28aab9ee
core-book3s.c 48.9 KB