On 05/16/2014 12:45 PM, Dave Hansen wrote: > From: Dave Hansen <[email protected]> > > 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/[email protected] > > 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 <[email protected]> > Cc: Thomas Gleixner <[email protected]> > Cc: [email protected] > Cc: Peter Zijlstra <[email protected]> > Cc: Gleb Natapov <[email protected]> > Cc: "H. Peter Anvin" <[email protected]> > Cc: [email protected] > Cc: Paolo Bonzini <[email protected]> > Cc: Steven Rostedt <[email protected]>
Acked-by: H. Peter Anvin <[email protected]> If Gleb and Paolo are okay with it, I am. -hpa -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
