On 05/08/2014 03:24 PM, Thomas Gleixner wrote:
>> > I noticed on some of my systems that page fault tracing doesn't work:
>> >
>> > cd /sys/kernel/debug/tracing
>> > echo 1 > events/exceptions/enable
>> > cat trace;
>> > # nothing shows up
>> >
>> > I eventually traced it down to CONFIG_KVM_GUEST. At least in a KVM VM,
>> > enabling that option breaks page fault tracing, and disabling fixes it.
>> > I tried on some old kernels and this does not appear to be a
>> > regression: it never worked.
>> >
>> > Anybody have any theories about what is going on?
Looks like the KVM code calls do_page_fault() directly:
> dotraplinkage void __kprobes
> do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
> {
> enum ctx_state prev_state;
>
> switch (kvm_read_and_reset_pf_reason()) {
> default:
> do_page_fault(regs, error_code);
> break;
> case KVM_PV_REASON_PAGE_NOT_PRESENT:
That seems to explain my problems in a VM. Any objections to doing
something like the attached patch?
---
b/arch/x86/include/asm/traps.h | 5 +++++
b/arch/x86/kernel/kvm.c | 2 +-
2 files changed, 6 insertions(+), 1 deletion(-)
diff -puN arch/x86/kernel/kvm.c~muck-with-kvm-guest-code arch/x86/kernel/kvm.c
--- a/arch/x86/kernel/kvm.c~muck-with-kvm-guest-code 2014-05-08 15:03:24.358110394 -0700
+++ b/arch/x86/kernel/kvm.c 2014-05-08 16:03:56.765302785 -0700
@@ -259,7 +259,7 @@ do_async_page_fault(struct pt_regs *regs
switch (kvm_read_and_reset_pf_reason()) {
default:
- do_page_fault(regs, error_code);
+ trace_do_page_fault(regs, error_code);
break;
case KVM_PV_REASON_PAGE_NOT_PRESENT:
/* page is swapped out by the host. */
diff -puN arch/x86/include/asm/traps.h~muck-with-kvm-guest-code arch/x86/include/asm/traps.h
--- a/arch/x86/include/asm/traps.h~muck-with-kvm-guest-code 2014-05-08 16:02:14.873675048 -0700
+++ b/arch/x86/include/asm/traps.h 2014-05-08 16:03:06.519020810 -0700
@@ -74,6 +74,11 @@ dotraplinkage void do_general_protection
dotraplinkage void do_page_fault(struct pt_regs *, unsigned long);
#ifdef CONFIG_TRACING
dotraplinkage void trace_do_page_fault(struct pt_regs *, unsigned long);
+#else
+static inline void trace_do_page_fault(struct pt_regs *regs, unsigned long error)
+{
+ do_page_fault(regs, error);
+}
#endif
dotraplinkage void do_spurious_interrupt_bug(struct pt_regs *, long);
dotraplinkage void do_coprocessor_error(struct pt_regs *, long);
_
