linux-kernel - Re: [PATCH v6 16/24] KVM: arm64: Add clock support for the pKVM hyp

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Message-ID: <aMQku2zzNRppTRhS@willie-the-truck>
Date: Fri, 12 Sep 2025 14:48:43 +0100
From: Will Deacon <will@...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,
	maz@...nel.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, aneesh.kumar@...nel.org,
	kernel-team@...roid.com, linux-kernel@...r.kernel.org,
	smostafa@...gle.com
Subject: Re: [PATCH v6 16/24] KVM: arm64: Add clock support for the pKVM hyp

[+Mostafa]

On Thu, Aug 21, 2025 at 09:14:04AM +0100, Vincent Donnefort wrote:
> By default, the arm64 host kernel is using the arch timer as a source
> for sched_clock. Conveniently, EL2 has access to that same counter,
> allowing to generate clock values that are synchronized.
> 
> The clock needs nonetheless to be setup with the same slope values as
> the kernel. Introducing at the same time trace_clock() which is expected
> to be later configured by the hypervisor tracing.
> 
> Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>

[...]

> +/* 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;
> +
> +	if (likely(cyc < clock->data[bank].cyc_overflow64)) {
> +		ns = cyc * clock->data[bank].mult;
> +		ns >>= clock->data[bank].shift;
> +	} else {
> +		ns = __clock_mult_uint128(cyc, clock->data[bank].mult,
> +					  clock->data[bank].shift);
> +	}
> +
> +	return (u64)ns + clock->data[bank].epoch_ns;

So the comment here say this is for debugging only but, afaict, this
would also be useful for the cmdq timeouts in your nested SMMU series [1]
and would remove the need to mess around with the clock frequency in
the driver code...

Will

[1] https://lore.kernel.org/lkml/20250819215156.2494305-4-smostafa@google.com/