linux-kernel - Re: [PATCH v8 20/28] KVM: arm64: Add clock support for the pKVM hyp

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Message-ID: <aR79MCNlAKP1H-PQ@google.com>
Date: Thu, 20 Nov 2025 11:36:16 +0000
From: Vincent Donnefort <vdonnefort@...gle.com>
To: Marc Zyngier <maz@...nel.org>, g@...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 20/28] KVM: arm64: Add clock support for the pKVM hyp

[...]

> > +/* Using host provided data. Do not use for anything else than debugging. */
> > +u64 trace_clock(void)
> > +{
> > +	struct clock_data *clock = &trace_clock_data;
> > +	u64 bank = smp_load_acquire(&clock->cur);
> > +	u64 cyc, ns;
> > +
> > +	cyc = __arch_counter_get_cntpct() - clock->data[bank].epoch_cyc;
> 
> I can only imagine that you don't care about the broken systems that
> do not have a monotonic counter, and that will happily have their
> counter swing back and forth?

I haven't used the _stable() variant for the affected devices... Hum this will
not be trivial from the hypervisor. I'll see what I can do.

> 
> Also, you may want to document why you are using the physical counter
> instead of the virtual one. I guess that you don't want CNTVOFF_EL2 to
> apply when HCR_EL2.E2H==0, but given that you never allow anything
> else for pKVM, this is slightly odd.

You mean I might as well use __arch_counter_get_cntvct() as CNTVOFF_EL2 will
always be ignored in the pKVM case?

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