On 05/16/2014 12:45 PM, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> 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.
>
> There are two page-fault entry functions today. One when tracing
> is on and another when it is off. The KVM code calls do_page_fault()
> directly instead of calling the traced version:
>
>> 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:
>
> I'm also having problems with the page fault tracing on bare
> metal (same symptom of no trace output). I'm unsure if it's
> related.
>
> Steven had an alternative to this which has zero overhead when
> tracing is off where this includes the standard noops even when
> tracing is disabled. I'm unconvinced that the extra complexity
> of his apporach:
>
> http://lkml.kernel.org/r/20140508194508.561ed220@xxxxxxxxxxxxxxxxxx
>
> is worth it, expecially considering that the KVM code is already
> making page fault entry slower here. This solution is
> dirt-simple.
>
> Gleb, please apply.
>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Gleb Natapov <gleb@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Acked-by: H. Peter Anvin <hpa@xxxxxxxxxxxxxxx>
If Gleb and Paolo are okay with it, I am.
-hpa
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