linux-kernel - Re: [PATCH v6] arch/powerpc/kvm: Add support for reading VPA counters for pseries guests

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Date: Thu, 09 May 2024 15:42:46 +1000
From: "Nicholas Piggin" <npiggin@...il.com>
To: "Michael Ellerman" <mpe@...erman.id.au>, "Gautam Menghani"
 <gautam@...ux.ibm.com>, <christophe.leroy@...roup.eu>,
 <naveen.n.rao@...ux.ibm.com>
Cc: <linuxppc-dev@...ts.ozlabs.org>, <kvm@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, "Vaibhav Jain" <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v6] arch/powerpc/kvm: Add support for reading VPA
 counters for pseries guests

On Wed May 8, 2024 at 10:36 PM AEST, Michael Ellerman wrote:
> Gautam Menghani <gautam@...ux.ibm.com> writes:
> > PAPR hypervisor has introduced three new counters in the VPA area of
> > LPAR CPUs for KVM L2 guest (see [1] for terminology) observability - 2
> > for context switches from host to guest and vice versa, and 1 counter
> > for getting the total time spent inside the KVM guest. Add a tracepoint
> > that enables reading the counters for use by ftrace/perf. Note that this
> > tracepoint is only available for nestedv2 API (i.e, KVM on PowerVM).
> ...
> > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> > index 8e86eb577eb8..ed69ad58bd02 100644
> > --- a/arch/powerpc/kvm/book3s_hv.c
> > +++ b/arch/powerpc/kvm/book3s_hv.c
> > @@ -4108,6 +4108,54 @@ static void vcpu_vpa_increment_dispatch(struct kvm_vcpu *vcpu)
> >  	}
> >  }
> >  
> > +static inline int kvmhv_get_l2_counters_status(void)
> > +{
> > +	return get_lppaca()->l2_counters_enable;
> > +}
>
> This is breaking the powernv build:

[...]

All the nested KVM code should really go under CONFIG_PSERIES.
Possibly even moved out to its own file.

For now maybe you could just ifdef these few functions and
replace with noop variants for !PSERIES.

Thanks,
Nick