Re: [PATCH v1 3/3] perf arm64: Support virtual CPU ID for kvm-stat

[Leo Yan <leo.yan@linaro.org>]

On Mon, Nov 07, 2022 at 03:39:07PM +0000, Marc Zyngier wrote:

[...]

> > > Please educate me: how useful is it to filter on a vcpu number across
> > > all VMs? What sense does it even make?
> > 
> > Now "perf kvm" tool is not sophisticated since it doesn't capture VMID
> > and virtual CPU ID together.
> 
> VMID is not a relevant indicator anyway, as it can change at any
> point.

Thanks for correction.  IIUC, VMID is not fixed for virtual machine, it
can be re-allocated after overflow.

> But that's only to show that everybody has a different view on
> what they need to collect. At which point, we need to provide an
> infrastructure for data extraction, and not the data itself.

Totally agree.

[...]

> > Option 3: As you suggested, I can bind KVM tracepoints with a eBPF
> > program and the eBPF program records perf events.
> > 
> > When I reviewed Arm64's kvm_entry / kvm_exit trace events, they don't
> > have vcpu context in the arguments, this means I need to add new trace
> > events for accessing "vcpu" context.
> 
> I'm not opposed to adding new trace{point,hook}s if you demonstrate
> that they are generic enough or will never need to evolve.
> 
> > 
> > Option 1 and 3 both need to add trace events; option 1 is more
> > straightforward solution and this is why it was choosed in current patch
> > set.
> > 
> > I recognized that I made a mistake, actually we can modify the trace
> > event's definition for kvm_entry / kvm_exit, note we only modify the
> > trace event's arguments, this will change the trace function's
> > definition but it will not break ABI (the format is exactly same for
> > the user space).  Below changes demonstrate what's my proposing:
> > 
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 94d33e296e10..16f6b61abfec 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -917,7 +917,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> >                 /**************************************************************
> >                  * Enter the guest
> >                  */
> > -               trace_kvm_entry(*vcpu_pc(vcpu));
> > +               trace_kvm_entry(vcpu);
> >                 guest_timing_enter_irqoff();
> >  
> >                 ret = kvm_arm_vcpu_enter_exit(vcpu);
> > diff --git a/arch/arm64/kvm/trace_arm.h b/arch/arm64/kvm/trace_arm.h
> > index 33e4e7dd2719..9df4fd30093c 100644
> > --- a/arch/arm64/kvm/trace_arm.h
> > +++ b/arch/arm64/kvm/trace_arm.h
> > @@ -12,15 +12,15 @@
> >   * Tracepoints for entry/exit to guest
> >   */
> >  TRACE_EVENT(kvm_entry,
> > -       TP_PROTO(unsigned long vcpu_pc),
> > -       TP_ARGS(vcpu_pc),
> > +       TP_PROTO(struct kvm_vcpu *vcpu),
> > +       TP_ARGS(vcpu),
> >  
> >         TP_STRUCT__entry(
> >                 __field(        unsigned long,  vcpu_pc         )
> >         ),
> >  
> >         TP_fast_assign(
> > -               __entry->vcpu_pc                = vcpu_pc;
> > +               __entry->vcpu_pc                = *vcpu_pc(vcpu);
> >         ),
> >  
> >         TP_printk("PC: 0x%016lx", __entry->vcpu_pc)
> > 
> > Please let me know your opinion, if you don't object, I can move
> > forward with this approach.
> 
> I have no issue with this if this doesn't change anything else.

Thanks for confirmation.

> And if you can make use of this with a BPF program and get to the same
> result as your initial patch, then please submit it for inclusion in
> the kernel as an example. We can then point people to it next time
> this crop up (probably before Xmas).

Will do.

Just head up, if everything is going well, I will place the eBPF
program in the folder tools/perf/examples/bpf/, this can be easy for
integration and release with perf tool.

Thanks,
Leo