[3.19.y-ckt stable] Patch "powerpc/perf: Fix book3s kernel to userspace backtraces" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    powerpc/perf: Fix book3s kernel to userspace backtraces

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.y-ckt4.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 8b5f9e3ebd431e9eb01fddfd8ccc917bd234ee6f Mon Sep 17 00:00:00 2001
From: Anton Blanchard <anton at samba.org>
Date: Tue, 26 May 2015 15:10:24 +1000
Subject: 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 at samba.org>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/powerpc/perf/core-book3s.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 7c4f669..3cb25fd 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -131,7 +131,16 @@ static void pmao_restore_workaround(bool ebb) { }

 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);
 }

 /*
--
1.9.1