Subject: Re: [PATCH] powerpc/perf: Fix book3s kernel to userspace backtraces

From: Laurentiu Tudor <[email-masked]>
Date: Tue, 26 May 2015 11:50:26 +0300

On 05/26/2015 08:10 AM, Anton Blanchard wrote:
> 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: Anton Blanchard <[email-masked]>
> ---
>  arch/powerpc/perf/core-book3s.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
> index 12b6384..42a3725 100644
> --- a/arch/powerpc/perf/core-book3s.c
> +++ b/arch/powerpc/perf/core-book3s.c
> @@ -131,7 +131,10 @@ static void pmao_restore_workaround(bool ebb) { }
>  
>  static bool regs_use_siar(struct pt_regs *regs)
>  {
> -	return !!regs->result;
> +	if ((TRAP(regs) == 0xf00) && regs->result)
> +		return true;
> +
> +	return false;

Why not just

return (TRAP(regs) == 0xf00) && regs->result;

?

---
Best Regards, Laurentiu;