linux-kernel - Re: [PATCH 1/4] arm64: hyperv: Add core Hyper-V include files

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Date:   Mon, 10 Dec 2018 18:43:27 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        "K . Y . Srinivasan" <kys@...rosoft.com>
Cc:     will.deacon@....com, catalin.marinas@...m.com,
        mark.rutland@....com, marc.zyngier@....com,
        linux-arm-kernel@...ts.infradead.org, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
        olaf@...fle.de, apw@...onical.com, jasowang@...hat.com,
        sthemmin@...rosoft.com
Subject: Re: [PATCH 1/4] arm64: hyperv: Add core Hyper-V include files

kys@...uxonhyperv.com writes:

> +
> +static inline u64 hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
> +				       u64 *cur_tsc)
> +{
> +	u64	scale, offset;
> +	u32	sequence;
> +
> +	/*
> +	 * The protocol for reading Hyper-V TSC page is specified in Hypervisor
> +	 * Top-Level Functional Specification.  To get the reference time we
> +	 * must do the following:
> +	 * - READ ReferenceTscSequence
> +	 *   A special '0' value indicates the time source is unreliable and we
> +	 *   need to use something else.
> +	 * - ReferenceTime =
> +	 *     ((HWclock val) * ReferenceTscScale) >> 64) + ReferenceTscOffset
> +	 * - READ ReferenceTscSequence again. In case its value has changed
> +	 *   since our first reading we need to discard ReferenceTime and repeat
> +	 *   the whole sequence as the hypervisor was updating the page in
> +	 *   between.
> +	 */
> +	do {
> +		sequence = READ_ONCE(tsc_pg->tsc_sequence);

(sorry if this was already discussed before)

Current x86 code doing this actually checks for '0' here (note the
comment about this special value above):

		sequence = READ_ONCE(tsc_pg->tsc_sequence);
		if (!sequence)
			return U64_MAX;

Was it removed intentionally (and we need to fix the comment then)? 

-- 
Vitaly