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

[Will Deacon <will@kernel.org>]

[+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@google.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/