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authorAnton Blanchard <anton@samba.org>2015-05-26 15:10:24 +1000
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2015-07-10 10:37:58 -0700
commit28aab9ee15e3c16ca55581e0336f29afca5310fd (patch)
treed7a554fd27981a58b0ddf6b1e9999c14afa22ad1
parent68ca9e96712a044dde8a872792282d22e3c3fdd6 (diff)
downloadlwn-28aab9ee15e3c16ca55581e0336f29afca5310fd.tar.gz
lwn-28aab9ee15e3c16ca55581e0336f29afca5310fd.zip
powerpc/perf: Fix book3s kernel to userspace backtraces
commit 72e349f1124a114435e599479c9b8d14bfd1ebcd 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: Anton Blanchard <anton@samba.org> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--arch/powerpc/perf/core-book3s.c11
1 files changed, 10 insertions, 1 deletions
diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 38265dc85318..65dfbd0c196d 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -124,7 +124,16 @@ static inline void power_pmu_bhrb_read(struct cpu_hw_events *cpuhw) {}
static bool regs_use_siar(struct pt_regs *regs)
{
- return !!regs->result;
+ /*
+ * When we take a performance monitor exception the regs are setup
+ * using perf_read_regs() which overloads some fields, in particular
+ * regs->result to tell us whether to use SIAR.
+ *
+ * However if the regs are from another exception, eg. a syscall, then
+ * they have not been setup using perf_read_regs() and so regs->result
+ * is something random.
+ */
+ return ((TRAP(regs) == 0xf00) && regs->result);
}
/*