[PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

[Stephen Hemminger]

Why not use existing seqlock's?

-----Original Message-----
From: Thomas Gleixner [mailto:tglx at linutronix.de] 
Sent: Thursday, February 9, 2017 9:08 AM
To: Vitaly Kuznetsov <vkuznets at redhat.com>
Cc: x86 at kernel.org; Andy Lutomirski <luto at amacapital.net>; Ingo Molnar <mingo at redhat.com>; H. Peter Anvin <hpa at zytor.com>; KY Srinivasan <kys at microsoft.com>; Haiyang Zhang <haiyangz at microsoft.com>; Stephen Hemminger <sthemmin at microsoft.com>; Dexuan Cui <decui at microsoft.com>; linux-kernel at vger.kernel.org; devel at linuxdriverproject.org; virtualization at lists.linux-foundation.org
Subject: Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

On Thu, 9 Feb 2017, Vitaly Kuznetsov wrote:
> +#ifdef CONFIG_HYPERV_TSCPAGE
> +static notrace u64 vread_hvclock(int *mode) {
> +	const struct ms_hyperv_tsc_page *tsc_pg =
> +		(const struct ms_hyperv_tsc_page *)&hvclock_page;
> +	u64 sequence, scale, offset, current_tick, cur_tsc;
> +
> +	while (1) {
> +		sequence = READ_ONCE(tsc_pg->tsc_sequence);
> +		if (!sequence)
> +			break;
> +
> +		scale = READ_ONCE(tsc_pg->tsc_scale);
> +		offset = READ_ONCE(tsc_pg->tsc_offset);
> +		rdtscll(cur_tsc);
> +
> +		current_tick = mul_u64_u64_shr(cur_tsc, scale, 64) + offset;
> +
> +		if (READ_ONCE(tsc_pg->tsc_sequence) == sequence)
> +			return current_tick;

That sequence stuff lacks still a sensible explanation. It's fundamentally different from the sequence counting we do in the kernel, so documentation for it is really required.

Thanks,

	tglx