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Date: Thu, 9 Feb 2017 18:08:22 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
cc: x86@kernel.org, Andy Lutomirski <luto@amacapital.net>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
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        Haiyang Zhang <haiyangz@microsoft.com>,
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        Dexuan Cui <decui@microsoft.com>, linux-kernel@vger.kernel.org,
        devel@linuxdriverproject.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read
 method
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References: <20170209141052.18694-1-vkuznets@redhat.com> <20170209141052.18694-3-vkuznets@redhat.com>
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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