RE: [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@xxxxxxxxxxxxx] 
Sent: Thursday, February 9, 2017 9:08 AM
To: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx; Andy Lutomirski <luto@xxxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; H. Peter Anvin <hpa@xxxxxxxxx>; KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>; Dexuan Cui <decui@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
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