RE: [RFC PATCH 2/8] clocksource/hyperv: Implement read_raw() for TSC page clocksource

[Michael Kelley]

From: David Woodhouse <dwmw2@xxxxxxxxxxxxx> Sent: Tuesday, May 26, 2026 4:06 PM
> 
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> 
> Implement the read_raw() callback for the Hyper-V TSC page
> clocksource. This returns the derived 10MHz reference time (for
> timekeeping) while also providing the raw TSC value that was used
> to compute it.
> 
> When the TSC page is valid, hv_read_tsc_page_tsc() atomically
> captures both values from a single RDTSC inside the sequence-counter
> protected read. When the TSC page is invalid (sequence == 0), raw is
> set to zero indicating no value is available.
> 
> This enables ktime_get_snapshot_id() to provide the raw TSC to
> consumers like KVM's master clock when running nested on Hyper-V.
> 
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Assisted-by: Kiro:claude-opus-4.6-1m

Looking narrowly at just the Hyper-V clocksource code in this patch:

Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>

> ---
>  drivers/clocksource/hyperv_timer.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index e9f5034a1bc8..c5ae01fdbd8e 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -444,6 +444,18 @@ static u64 notrace read_hv_clock_tsc_cs(struct clocksource *arg)
>  	return read_hv_clock_tsc();
>  }
> 
> +static u64 notrace read_hv_clock_tsc_cs_raw(struct clocksource *arg, u64 *raw)
> +{
> +	u64 time;
> +
> +	if (!hv_read_tsc_page_tsc(tsc_page, raw, &time)) {
> +		time = read_hv_clock_msr();
> +		*raw = 0;
> +	}
> +
> +	return time;
> +}
> +
>  static u64 noinstr read_hv_sched_clock_tsc(void)
>  {
>  	return (read_hv_clock_tsc() - hv_sched_clock_offset) *
> @@ -495,6 +507,8 @@ static struct clocksource hyperv_cs_tsc = {
>  	.name	= "hyperv_clocksource_tsc_page",
>  	.rating	= 500,
>  	.read	= read_hv_clock_tsc_cs,
> +	.read_raw = read_hv_clock_tsc_cs_raw,
> +	.raw_csid = CSID_X86_TSC,
>  	.mask	= CLOCKSOURCE_MASK(64),
>  	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
>  	.suspend= suspend_hv_clock_tsc,
> --
> 2.54.0
>