Re: [PATCH] x86: fix page fault tracing when KVM guest support enabled

[Paolo Bonzini]

Il 16/05/2014 22:53, H. Peter Anvin ha scritto:
> 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.

Yes, of course.  Dave, ok to only have it in 3.16?

Paolo

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/