linux-kernel - RE: [PATCH 2/2] x86/vdso: Add VCLOCK

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Message-ID: <BLUPR0301MB20986D17696C3E658D511FACCC450@BLUPR0301MB2098.namprd03.prod.outlook.com>
Date:   Thu, 9 Feb 2017 18:27:27 +0000
From:   Stephen Hemminger <sthemmin@...rosoft.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "KY Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

Why not use existing seqlock's?

-----Original Message-----
From: Thomas Gleixner [mailto:tglx@...utronix.de] 
Sent: Thursday, February 9, 2017 9:08 AM
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: x86@...nel.org; Andy Lutomirski <luto@...capital.net>; Ingo Molnar <mingo@...hat.com>; H. Peter Anvin <hpa@...or.com>; KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang <haiyangz@...rosoft.com>; Stephen Hemminger <sthemmin@...rosoft.com>; Dexuan Cui <decui@...rosoft.com>; linux-kernel@...r.kernel.org; devel@...uxdriverproject.org; virtualization@...ts.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