linux-kernel - Re: [PATCH v8 26/28] KVM: arm64: Add hyp_enter/hyp

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Message-ID: <86o6oip3fr.wl-maz@kernel.org>
Date: Mon, 01 Dec 2025 16:33:28 +0000
From: Marc Zyngier <maz@...nel.org>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: rostedt@...dmis.org,
	mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com,
	linux-trace-kernel@...r.kernel.org,
	oliver.upton@...ux.dev,
	joey.gouly@....com,
	suzuki.poulose@....com,
	yuzenghui@...wei.com,
	kvmarm@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	jstultz@...gle.com,
	qperret@...gle.com,
	will@...nel.org,
	aneesh.kumar@...nel.org,
	kernel-team@...roid.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 26/28] KVM: arm64: Add hyp_enter/hyp_exit events to pKVM hyp

On Mon, 01 Dec 2025 16:04:07 +0000,
Vincent Donnefort <vdonnefort@...gle.com> wrote:
> 
> On Sun, Nov 30, 2025 at 07:00:52PM +0000, Marc Zyngier wrote:
> > On Fri, 07 Nov 2025 09:38:38 +0000,
> > Vincent Donnefort <vdonnefort@...gle.com> wrote:
> > > 
> > > The hyp_enter and hyp_exit events are logged by the hypervisor any time
> > > it is entered and exited.
> > > 
> > > Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>
> > > 
> > > diff --git a/arch/arm64/include/asm/kvm_hypevents.h b/arch/arm64/include/asm/kvm_hypevents.h
> > > index d6e033c96c52..ce3953bc884a 100644
> > > --- a/arch/arm64/include/asm/kvm_hypevents.h
> > > +++ b/arch/arm64/include/asm/kvm_hypevents.h
> > > @@ -7,4 +7,21 @@
> > >  #include <nvhe/trace.h>
> > >  #endif
> > >  
> > > +HYP_EVENT(hyp_enter,
> > > +	HE_PROTO(void),
> > > +	HE_STRUCT(
> > > +	),
> > > +	HE_ASSIGN(
> > > +	),
> > > +	HE_PRINTK()
> > > +);
> > > +
> > > +HYP_EVENT(hyp_exit,
> > > +	HE_PROTO(void),
> > > +	HE_STRUCT(
> > > +	),
> > > +	HE_ASSIGN(
> > > +	),
> > > +	HE_PRINTK()
> > > +);
> > >  #endif
> > > diff --git a/arch/arm64/kvm/hyp/include/nvhe/arm-smccc.h b/arch/arm64/kvm/hyp/include/nvhe/arm-smccc.h
> > > new file mode 100644
> > > index 000000000000..4b69d33e4f2d
> > > --- /dev/null
> > > +++ b/arch/arm64/kvm/hyp/include/nvhe/arm-smccc.h
> > > @@ -0,0 +1,13 @@
> > > +/* SPDX-License-Identifier: GPL-2.0-only */
> > > +
> > > +#include <asm/kvm_hypevents.h>
> > > +
> > > +#include <linux/arm-smccc.h>
> > > +
> > > +#undef arm_smccc_1_1_smc
> > > +#define arm_smccc_1_1_smc(...)					\
> > > +	do {							\
> > > +		trace_hyp_exit();				\
> > > +		__arm_smccc_1_1(SMCCC_SMC_INST, __VA_ARGS__);	\
> > > +		trace_hyp_enter();				\
> > > +	} while (0)
> > 
> > Huh. No, please. Don't override existing kernel constructs behind
> > everybody's back. I know that tracing folks love to play that sort of
> > game, but I really don't want any of that here.
> 
> And you haven't seen what I had for arm_smccc_1_2_smc()

I have the feeling that I don't want to see it at all!

> > Just define a wrapper that is KVM-specific, and use that.
> > 
> > Then from a semantic perspective, an SMC call, an ERET back to EL1
> > host, and an ERET back to EL1 guest are all very different events that
> > you lump under the "exit" category. That's not right. Same thing for
> > "enter".
> 
> The idea of those events is just to know when we enter and exit the hypervisor,
> purely from a timing point of view. In ACK, we already have other separated
> events that can give the "cause" for entering or exiting the hypervisor. I
> haven't added them to that series because I thought it was big enough.
> 
> So happy to add now or do a follow-up, once we're done with that one?

Fixed trace points are ABI AFAIC, so I'd rather we make them useful
from the very start. What you have here is not very interesting,
because it makes all entry and exit points equal -- they very much
aren't.

So get rid of these and implement the real thing.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.