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

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Message-ID: <878qfnfkud.wl-maz@kernel.org>
Date: Sun, 30 Nov 2025 18:15:22 +0000
From: Marc Zyngier <maz@...nel.org>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: g@...gle.com,
	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

On Thu, 20 Nov 2025 11:36:16 +0000,
Vincent Donnefort <vdonnefort@...gle.com> wrote:
> 
> [...]
> 
> > > +/* 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.

In all honestly, simply disabling tracing when you don't have a stable
counter should be enough. I don't think there is much to be gained
with supporting that on sub-standard HW (the bar is low enough).

> > 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?

That, plus the fact that CNTVOFF_EL2 is only evaluated at EL2 when
E2H=0 (and that case is going to disappear over time).

	M.

-- 
Jazz isn't dead. It just smells funny.