Re: [PATCH v2 1/3] clocksource/hyperv: Implement read_snapshot() for TSC page clocksource

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] This problem wasn't introduced by this patch, but if `memremap` fails during Hyper-V root partition initialization, the active clocksource is not unregistered and `tsc_page` becomes NULL, leading to a guaranteed kernel panic upon the next clock read.
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commit fd7e137ffdb89db4afcdcc509ba544a534823492
Author: David Woodhouse <dwmw@amazon.co.uk>

clocksource/hyperv: Implement read_snapshot() for TSC page clocksource

This commit implements the read_snapshot() callback for the Hyper-V TSC
page clocksource. It allows the raw TSC value and the derived time to be
atomically captured, enabling ktime_get_snapshot_id() to provide the raw
TSC to consumers like KVM's master clock.

> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
> index e9f5034a1bc89..df567795d1751 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c

[ ... ]

> @@ -444,6 +444,22 @@ static u64 notrace read_hv_clock_tsc_cs(struct clocksource *arg)
>  	return read_hv_clock_tsc();
>  }
>  
> +static u64 notrace read_hv_clock_tsc_cs_snapshot(struct clocksource *arg,
> +						  struct clocksource_hw_snapshot *chs)
> +{
> +	u64 time;
> +
> +	if (hv_read_tsc_page_tsc(tsc_page, &chs->hw_cycles, &time)) {

[Severity: High]
This isn't a bug introduced by this patch, but if memremap() fails during
Hyper-V root partition initialization, tsc_page becomes NULL:

drivers/clocksource/hyperv_timer.c:hv_remap_tsc_clocksource() {
	...
	tsc_page = memremap(tsc_pfn << HV_HYP_PAGE_SHIFT, sizeof(tsc_pg),
			    MEMREMAP_WB);
	if (!tsc_page)
		pr_err("Failed to remap Hyper-V TSC page.\n");
	...
}

Since the clocksource is not unregistered and a valid fallback pointer isn't
restored on failure, will passing this NULL tsc_page to hv_read_tsc_page_tsc()
cause a kernel panic upon the next snapshot read?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260604095755.64849-1-dwmw2@infradead.org?part=1