Re: [PATCH v3 2/4] arm64: KVM: add accessors to track guest/host only counters

From: Will Deacon <will.deacon@arm.com>
To: Andrew Murray <andrew.murray@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 2/4] arm64: KVM: add accessors to track guest/host only counters
Date: Mon, 3 Dec 2018 16:58:43 +0000	[thread overview]
Message-ID: <20181203165842.GB27796@arm.com> (raw)
In-Reply-To: <20181203150551.GC16311@e119886-lin.cambridge.arm.com>

On Mon, Dec 03, 2018 at 03:05:52PM +0000, Andrew Murray wrote:
> On Mon, Dec 03, 2018 at 02:31:14PM +0000, Will Deacon wrote:
> > On Thu, Nov 22, 2018 at 11:25:48AM +0000, Andrew Murray wrote:
> > > In order to effeciently enable/disable guest/host only perf counters
> > > at guest entry/exit we add bitfields to kvm_cpu_context for guest and
> > > host only events as well as accessors for updating them.
> > > 
> > > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > > ---
> > >  arch/arm64/include/asm/kvm_host.h | 20 ++++++++++++++++++++
> > >  1 file changed, 20 insertions(+)
> > > 
> > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > > index 1550192..4a828eb 100644
> > > --- a/arch/arm64/include/asm/kvm_host.h
> > > +++ b/arch/arm64/include/asm/kvm_host.h
> > > @@ -203,6 +203,8 @@ struct kvm_cpu_context {
> > >  	};
> > >  
> > >  	struct kvm_vcpu *__hyp_running_vcpu;
> > > +	u32 events_host_only;
> > > +	u32 events_guest_only;
> > >  };
> > >  
> > >  typedef struct kvm_cpu_context kvm_cpu_context_t;
> > > @@ -472,6 +474,24 @@ static inline int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
> > >  {
> > >  	return kvm_arch_vcpu_run_map_fp(vcpu);
> > >  }
> > > +static inline void kvm_clr_set_host_pmu_events(u32 clr, u32 set)
> > > +{
> > > +	kvm_cpu_context_t *ctx = this_cpu_ptr(&kvm_host_cpu_state);
> > > +
> > > +	ctx->events_host_only &= ~clr;
> > > +	ctx->events_host_only |= set;
> > > +}
> > 
> > Do you actually need this clr/set function, or can you just use the
> > facilities provided by bitfield.h at the callsites?
> 
> This modifies events_{host|guest}_only which is conditionally defined
> (CONFIG_KVM) yet used by arch/arm64/kernel/perf_events which is also
> conditionally defined (CONFIG_HW_PERF_EVENTS). By moving this away
> from the callsite we avoid a load of horrible #ifdef's and allow this
> function to become a nop when KVM isn't enabled.
> 
> Is there value in updating this function to use set_bit/clear_bit
> functions in bitops/atomic.h ?

You don't need the atomicity, so you'd be looking at the '__' variants
instead. My main concern is that we're introducing a clr_set_* API which
only ever clears or sets, so I think it's needlessly complicated. If you
need two separate macros that are just #defined to __set_bit/__clear_bit
or expand to nothing then that could work too.

Will